This just works at the moment after e19892a099, but if we ever do
the FIXME in ColorIndexingTransform::transform(), this test will
remind us to think of this case there too.
catdog-alert-13-alpha-used-false.webp is identical to
catdog-alert-13.web but with the byte at offset 0x2a changed from
0x10 to 0x00 -- that is, the bit in the VP8L header that stores
`is_alpha_used` is cleared.
See the commit message of e19892a099 for more information.
