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.

cjpeg.1

@@ -1,4 +1,4 @@
-.TH CJPEG 1 "27 April 1999"
+.TH CJPEG 1 "11 November 2022"
 .SH NAME
 cjpeg \- compress an image file to a JPEG file
 .SH SYNOPSIS
@@ -62,13 +62,6 @@ decompression are unaffected by
 .B \-progressive
 Create progressive JPEG file (see below).
 .TP
-.BI \-lossless " psv[,Pt]"
-Create a lossless JPEG file using the specified predictor selection value (1-7)
-and optional point transform.
-.B Caution:
-lossless JPEG is not widely implemented, so many decoders will be
-unable to view a lossless JPEG file at all.
-.TP
 .B \-targa
 Input file is Targa format.  Targa files that contain an "identification"
 field will not be automatically recognized by
@@ -130,6 +123,43 @@ unable to view a progressive JPEG file at all.
 .PP
 Switches for advanced users:
 .TP
+.BI \-lossless " psv[,Pt]"
+Create a lossless JPEG file using the specified predictor selection value
+(1 through 7) and optional point transform (0 through
+.nh
+.I precision
+.hy
+- 1, where
+.nh
+.I precision
+.hy
+is the JPEG data precision in bits).  A point transform value of 0 (the
+default) is necessary in order to create a fully lossless JPEG file.  (A
+non-zero point transform value right-shifts the input samples by the specified
+number of bits, which is effectively a form of lossy color quantization.)
+.B Caution:
+lossless JPEG is not yet widely implemented, so many decoders will be unable to
+view a lossless JPEG file at all.  Note that the following features will be
+unavailable when compressing or decompressing a lossless JPEG file:
+.IP
+- Quality/quantization table selection
+.IP
+- Color conversion (the JPEG image will use the same color space as the input
+image)
+.IP
+- DCT/IDCT algorithm selection
+.IP
+- Smoothing
+.IP
+- Downsampling/upsampling
+.IP
+- IDCT scaling
+.IP
+- Transformations using
+.B jpegtran
+.IP
+Any switches used to enable or configure those features will be ignored.
+.TP
 .B \-dct int
 Use integer DCT method (default).
 .TP
@@ -145,8 +175,8 @@ machines, while the integer methods should give the same results everywhere.
 The fast integer method is much less accurate than the other two.
 .TP
 .BI \-restart " N"
-Emit a JPEG restart marker every N MCU rows, or every N MCU blocks if "B" is
-attached to the number.
+Emit a JPEG restart marker every N MCU rows, or every N MCU blocks (samples in
+lossless mode) if "B" is attached to the number.
 .B \-restart 0
 (the default) means no restart markers.
 .TP