update (non-sprite) image shader color management

Just another little step...  diffs down to ~450 from ~500.

The A8 handling has changed a little, keeping the paint
color in sRGB just to make the append_misc() logic a bit
more streamlined.

I may want to refactor things a little later so that I can
get an easy signal to use to_srgb and from_srgb stages instead
of sending them through the generic parametric stages.

Change-Id: Iefff3206b06a3aa7d81f85fd169272b6ac9c20e0
Reviewed-on: https://skia-review.googlesource.com/140250
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
5 files changed