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.

jdcolor.c

@@ -1,8 +1,10 @@
 /*
  * jdcolor.c
  *
+ * This file was part of the Independent JPEG Group's software:
  * Copyright (C) 1991-1997, Thomas G. Lane.
- * This file is part of the Independent JPEG Group's software.
+ * Lossless JPEG Modifications:
+ * Copyright (C) 2022, D. R. Commander.
  * For conditions of distribution and use, see the accompanying README file.
  *
  * This file contains output colorspace conversion routines.
@@ -340,10 +342,15 @@ jinit_color_deconverter (j_decompress_ptr cinfo)
   /* Set out_color_components and conversion method based on requested space.
    * Also clear the component_needed flags for any unused components,
    * so that earlier pipeline stages can avoid useless computation.
+   * NOTE: We do not allow any lossy color conversion algorithms in lossless
+   * mode.
    */
 
   switch (cinfo->out_color_space) {
   case JCS_GRAYSCALE:
+    if (cinfo->master->lossless &&
+	cinfo->jpeg_color_space != cinfo->out_color_space)
+      ERREXIT(cinfo, JERR_CONVERSION_NOTIMPL);
     cinfo->out_color_components = 1;
     if (cinfo->jpeg_color_space == JCS_GRAYSCALE ||
 	cinfo->jpeg_color_space == JCS_YCbCr) {
@@ -356,6 +363,9 @@ jinit_color_deconverter (j_decompress_ptr cinfo)
     break;
 
   case JCS_RGB:
+    if (cinfo->master->lossless &&
+	cinfo->jpeg_color_space != JCS_RGB)
+      ERREXIT(cinfo, JERR_CONVERSION_NOTIMPL);
     cinfo->out_color_components = RGB_PIXELSIZE;
     if (cinfo->jpeg_color_space == JCS_YCbCr) {
       cconvert->pub.color_convert = ycc_rgb_convert;
@@ -369,6 +379,9 @@ jinit_color_deconverter (j_decompress_ptr cinfo)
     break;
 
   case JCS_CMYK:
+    if (cinfo->master->lossless &&
+	cinfo->jpeg_color_space != cinfo->out_color_space)
+      ERREXIT(cinfo, JERR_CONVERSION_NOTIMPL);
     cinfo->out_color_components = 4;
     if (cinfo->jpeg_color_space == JCS_YCCK) {
       cconvert->pub.color_convert = ycck_cmyk_convert;