Support mixing shaders and color filters in runtime effects

Needs more testing, but includes a GM that demonstrates the ultimate
benefit of this: our 3D color LUT demo working as a color filter.

Bug: skia:11813
Change-Id: I97c129c54bcf2cb788c0806b5d9e907ff058bb69
Reviewed-by: Mike Klein <>
Reviewed-by: Michael Ludwig <>
Reviewed-by: Brian Salomon <>
Commit-Queue: Brian Osman <>
3 files changed