allow SkRP color filters to fail

I think we decided at some point that we had implemented all existing
color filters with SkRasterPipeline, and then stopped checking for
appendStages() failure.  It's all still plumbed through, and I'd like to
start making some color filters fail to appendStage() and use
SkVMBlitter, at least some of the time.

Change-Id: I520151e7bfc01c4cf333a6ed515f27bee6cd55ee
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/308195
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
5 files changed