This adds `shader && paint alpha < 1.0f` to the blitter Key, so we'll
generate and cache two different programs based on that bit.
I put a little bit of effort into a different approach, wrapping the
shader with a new shader that modulates by paint alpha when paint alpha
is less than one. This should work, but I realized that it would break
caching, since we're caching based on shader identity. (Not really
break, but make it ineffective.)
I've added some notes about cache keys in general, and that this wrapper
approach would make caching less effective is I think enough to make me
want to work out a "identify yourself except for uniforms" protocol for
shaders (really effects, eventually color filters will need it too).
Change-Id: I1a2a490657bf7442e79f87d80968302720265f7d
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/251775
Reviewed-by: Mike Klein <mtklein@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
2eae8e53ba