If a lossless webp has 3 or 4 colors, it uses 2 bits per pixel to
store an offset into a "color index" (which the spec explicitly does
not call palette since it says the 'color cache' is more like that).
This way, it can pack 4 pixels into a single pixel.
If the width of the output image wasn't evenly divisble by 4,
we used to write out-of-bounds in the last few columns of each
row, since we used to always write all 4 pixels.
Found by clusterfuzz. Probably fixes
https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=66082
While here, spruce up the comments very slightly.
simple-vp8l-alpha-used-false.webp is a copy of simple-vp8l.webp,
with the byte at offset 0x18 changed from 0x10 to 0x00 -- that
is, the bit in the VP8L header that stores `is_alpha_used` is cleared.
We would already allocated a BGRx8888 instead of a BGRA8888 bitmap,
but keep actual alpha data in the `x` channel.
That lead to at least `image` still writing a PNG with an alpha channel.
So explicitly set the alpha channel to 0xff when is_alpha_used is false,
to make sure all consumers of decoded lossless webp data have behavior
consistent with other webp readers.
In practice, webp encoders usually don't write files that have
`is_alpha_used` set to false and then write actual alpha data to their
output. So this is rarely observable. However, for example for
lossy+ALPH webp files, the lossless webp used to store the ALPH channel
has `is_alpha_used` set to false and all channels but green are 0
(since the lossless green channel stores the alpha channel of a
lossy+ALPH webp). So if we dump such a bitmap to a standalone webp
file (e.g. with the temporary debugging code in fc3249a1ca),
then without this commit here, `image` would convert that webp to
a fully transparent webp, while other webp software would correctly
display the green image with opaque alpha.
