WebP: More precise alpha-blending for some boundary cases

When alpha value of src is 0, we simply see-through the dst
pixel. That way we avoid computation and rounding error for
this (common) boundary case.

This also covers the blendA = 0 case, as blendA = 0 if and
only if srcA = 0 and dstA = 0.

Additionally, ensure that dstFactorA is always less than 256 (guaranteed by the code).

Reviwers:
skal: For core logic
Nico: For OWNER'S lgtm stamp.

BUG=364679

Review URL: https://codereview.chromium.org/298013002

git-svn-id: svn://svn.chromium.org/blink/trunk@174771 bbb929c8-8fbe-4397-9dbb-9b2b20218538