Commit 9d4084c82f skips conversion and
blending of (some) transparent source and destination pixels. When
`blend_out == blend_layer`, it banks on the fact that the alpha values
of `blend_out` would be the same as those of `blend_layer`, and hence
the same as those of `layer`; thing is, we only copy those values from
`layer` to `blend_layer` for the pixels that we *don't* skip, so this
assumption is just wrong :P This leaves us with bogus alpha values in
`blend_out` for the skipped pixels, when the above equality holds.
For composite modes that use the alpha values of `blend_op` (aka `comp`)
even for transparent input pixels (i.e., src-atop and src-in), this may
result in artifacts.
Fix this by simply initializing the alpha values of `blend_out` for
skipped pixels unconditionally.
2d22d0b0ff