WebP lossless differs from deflate in how it handles 1-element codes.
Deflate consumes one bit from the bitstream to produce the element,
while webp lossless consumes 0 bits. Add a wrapper class to handle
this case.
The `static` here meant we always kept the alphabet sizes of the
first image we happened to load -- and a single webp lossless image
can store several helper images used during decoding.
Usually, the helper images wouldn't use a color cache but the main
image would, but the main image would then use the first entry from
the helper images due to the `static`, which led us to not decoding
the codes for the color cache symbols.
The spec is at best misleading here, suggesting that max_symbol should
be set to "num_code_lengths" if it's not explicitly stored.
But num_code_lengths doesn't mean the num_code_lengths mentioned a few
lines further up in the spec, but alphabet_size!
(I had to cheat and look at libwebp instead of the spec for this: See
vp8l_dec.c, ReadHuffmanCode() which passes alphabet_size to
ReadHuffmanCodeLengths() as num_symbols, and ReadHuffmanCodeLengths()
then sets max_symbol to that.)
I haven't yet found a file that uses max_symbol, so this isn't actually
tested. But it's close to what's in libwebp, so maybe it works!
Missing:
* Transform support (used by virtually all lossless webp files)
* Meta prefix / entropy image support
Working:
* Decoding of regular image streams
* Color cache
This happens to be enough to be able to decode
Tests/LibGfx/test-inputs/extended-lossless.webp
The canonical prefix code is very similar to deflate's, enough so that
this can use Compress::CanonicalCode (and take advantage of all the
recent performance improvements there).
