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).
