Change PixelsAreSimilar() to handle black pixels correctly.
Change PixelsAreSimilar() to handle black pixels correctly.

if src_{r,g,b} = 0, any value of src_a or dst_a such that
   abs(src_a - dst_a) <= max_allowed_diff
was making the test pass, despite being very different-looking pixels.

The fix is to require same values for src_a and dst_a before attempting
an r/g/b comparison.

Change-Id: If3a55a229eab3904ed454f20065e49e35c39f25c

PKRoma/libwebp,webmproject/libwebp,PKRoma/libwebp,PKRoma/libwebp,webmproject/libwebp,PKRoma/libwebp,webmproject/libwebp,webmproject/libwebp,PKRoma/libwebp,PKRoma/libwebp,webmproject/libwebp