Nico Weber 0d2e6162db LibGfx/WebP: Implement compressed ALPH chunk reading ...

A compressed ALPH chunk is a lossless webp bitstream, but without
the 5 byte "header" that stores width, height, is-alpha-channel-used
(it never is for an ALPH chunk since the ALPH chunk gets the alpha
data out of the lossless webp's green channel), and version fields.

For that reason, this cuts decode_webp_chunk_VP8L() into the
header-reading part and the remaining part, so that the remaining
part can be called by the ALPH reading routine.

Lossy webp files with a (losslessly) compressed alpha channel can
be found in the wild. Since we can't decode lossy webp data yet,
change the `#if 0` in decode_webp_chunk_VP8() to `#if 1` to test this.

2023-05-07 07:08:05 +02:00

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