add two more cases to P3 gm

These are a sprite and non-sprite A8 image draw,
where we pick up the color from the paint.

I'm slightly disturbed to see these working.
It turns out I had no idea how A8 draws happened.

There is probably some dead code to remove in
SkBlitter_Sprite (I think, never called for A8)
and in SkImageShader (only ever called with a black
paint color for A8), in this CL as TODOs.

Change-Id: I1d276f8d9b145b57e3ab793735fb184bdae670fb
Reviewed-on: https://skia-review.googlesource.com/154461
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
3 files changed