Clean up the lossless JPEG feature

- Rename jpeg_simple_lossless() to jpeg_enable_lossless() and modify the
  function so that it stores the lossless parameters directly in the Ss
  and Al fields of jpeg_compress_struct rather than using a scan script.

- Move the cjpeg -lossless switch into "Switches for advanced users".

- Document the libjpeg API and run-time features that are unavailable in
  lossless mode, and ensure that all parameters, functions, and switches
  related to unavailable features are ignored or generate errors in
  lossless mode.

- Defer any action that depends on whether lossless mode is enabled
  until jpeg_start_compress()/jpeg_start_decompress() is called.

- Document the purpose of the point transform value.

- "Codec" stands for coder/decoder, so it is a bit awkward to say
  "lossless compression codec" and "lossless decompression codec".
  Use "lossless compressor" and "lossless decompressor" instead.

- Restore backward API/ABI compatibility with libjpeg v6b:

  * Move the new 'lossless' field from the exposed jpeg_compress_struct
    and jpeg_decompress_struct structures into the opaque
    jpeg_comp_master and jpeg_decomp_master structures, and allocate the
    master structures in the body of jpeg_create_compress() and
    jpeg_create_decompress().

  * Remove the new 'process' field from jpeg_compress_struct and
    jpeg_decompress_struct and replace it with the old
    'progressive_mode' field and the new 'lossless' field.

  * Remove the new 'data_unit' field from jpeg_compress_struct and
    jpeg_decompress_struct and replace it with a locally-computed
    data unit variable.

  * Restore the names of macros and fields that refer to DCT blocks, and
    document that they have a different meaning in lossless mode.  (Most
    of them aren't very meaningful in lossless mode anyhow.)

  * Remove the new alloc_darray() method from jpeg_memory_mgr and
    replace it with an internal macro that wraps the alloc_sarray()
    method.

  * Move the JDIFF* data types from jpeglib.h and jmorecfg.h into
    jpegint.h.

  * Remove the new 'codec' field from jpeg_compress_struct and
    jpeg_decompress_struct and instead reuse the existing internal
    coefficient control, forward/inverse DCT, and entropy
    encoding/decoding structures for lossless compression/decompression.

  * Repurpose existing error codes rather than introducing new ones.
    (The new JERR_BAD_RESTART and JWRN_MUST_DOWNSCALE codes remain,
    although JWRN_MUST_DOWNSCALE will probably be removed in
    libjpeg-turbo, since we have a different way of handling multiple
    data precisions.)

- Automatically enable lossless mode when a scan script with parameters
  that are only valid for lossless mode is detected, and document the
  use of scan scripts to generate lossless JPEG images.

- Move the sequential and shared Huffman routines back into jchuff.c and
  jdhuff.c, and document that those routines are shared with jclhuff.c
  and jdlhuff.c as well as with jcphuff.c and jdphuff.c.

- Move MAX_DIFF_BITS from jchuff.h into jclhuff.c, the only place where
  it is used.

- Move the predictor and scaler code into jclossls.c and jdlossls.c.

- Streamline register usage in the [un]differencers (inspired by similar
  optimizations in the color [de]converters.)

- Restructure the logic in a few places to reduce duplicated code.

- Ensure that all lossless-specific code is guarded by
  C_LOSSLESS_SUPPORTED or D_LOSSLESS_SUPPORTED and that the library can
  be built successfully if either or both of those macros is undefined.

- Remove all short forms of external names introduced by the lossless
  JPEG patch.  (These will not be needed by libjpeg-turbo, so there is
  no use cleaning them up.)

- Various wordsmithing, formatting, and punctuation tweaks

- Eliminate various compiler warnings.

jcmarker.c

@@ -5,6 +5,7 @@
  * Copyright (C) 1991-1998, Thomas G. Lane.
  * Lossless JPEG Modifications:
  * Copyright (C) 1999, Ken Murchison.
+ * Copyright (C) 2022, D. R. Commander.
  * For conditions of distribution and use, see the accompanying README file.
  *
  * This file contains routines to write JPEG datastream markers.
@@ -324,7 +325,7 @@ emit_sos (j_compress_ptr cinfo)
     emit_byte(cinfo, compptr->component_id);
     td = compptr->dc_tbl_no;
     ta = compptr->ac_tbl_no;
-    if (cinfo->process == JPROC_PROGRESSIVE) {
+    if (cinfo->progressive_mode) {
       /* Progressive mode: only DC or only AC tables are used in one scan;
        * furthermore, Huffman coding of DC refinement uses no table at all.
        * We emit 0 for unused field(s); this is recommended by the P&M text
@@ -495,15 +496,14 @@ write_file_header (j_compress_ptr cinfo)
 METHODDEF(void)
 write_frame_header (j_compress_ptr cinfo)
 {
-  int ci, prec;
+  int ci, prec = 0;
   boolean is_baseline;
   jpeg_component_info *compptr;
 
-  if (cinfo->process != JPROC_LOSSLESS) {
+  if (! cinfo->master->lossless) {
     /* Emit DQT for each quantization table.
      * Note that emit_dqt() suppresses any duplicate tables.
      */
-    prec = 0;
     for (ci = 0, compptr = cinfo->comp_info; ci < cinfo->num_components;
 	 ci++, compptr++) {
       prec += emit_dqt(cinfo, compptr->quant_tbl_no);
@@ -514,8 +514,8 @@ write_frame_header (j_compress_ptr cinfo)
   /* Check for a non-baseline specification.
    * Note we assume that Huffman table numbers won't be changed later.
    */
-  if (cinfo->arith_code || cinfo->process != JPROC_SEQUENTIAL ||
-      cinfo->data_precision != 8) {
+  if (cinfo->arith_code || cinfo->progressive_mode ||
+      cinfo->master->lossless || cinfo->data_precision != 8) {
     is_baseline = FALSE;
   } else {
     is_baseline = TRUE;
@@ -535,9 +535,9 @@ write_frame_header (j_compress_ptr cinfo)
   if (cinfo->arith_code) {
     emit_sof(cinfo, M_SOF9);	/* SOF code for arithmetic coding */
   } else {
-    if (cinfo->process == JPROC_PROGRESSIVE)
+    if (cinfo->progressive_mode)
       emit_sof(cinfo, M_SOF2);	/* SOF code for progressive Huffman */
-    else if (cinfo->process == JPROC_LOSSLESS)
+    else if (cinfo->master->lossless)
       emit_sof(cinfo, M_SOF3);	/* SOF code for lossless Huffman */
     else if (is_baseline)
       emit_sof(cinfo, M_SOF0);	/* SOF code for baseline implementation */
@@ -572,7 +572,7 @@ write_scan_header (j_compress_ptr cinfo)
      */
     for (i = 0; i < cinfo->comps_in_scan; i++) {
       compptr = cinfo->cur_comp_info[i];
-      if (cinfo->process == JPROC_PROGRESSIVE) {
+      if (cinfo->progressive_mode) {
 	/* Progressive mode: only DC or only AC tables are used in one scan */
 	if (cinfo->Ss == 0) {
 	  if (cinfo->Ah == 0)	/* DC needs no table for refinement scan */
@@ -580,7 +580,7 @@ write_scan_header (j_compress_ptr cinfo)
 	} else {
 	  emit_dht(cinfo, compptr->ac_tbl_no, TRUE);
 	}
-      } else if (cinfo->process == JPROC_LOSSLESS) {
+      } else if (cinfo->master->lossless) {
 	/* Lossless mode: only DC tables are used */
 	emit_dht(cinfo, compptr->dc_tbl_no, FALSE);
       } else {