Update GrFragmentProcessor::SwizzleOutput to use a child FP.

We are updating FPs to receive their input via a child FP where
possible, instead of relying on the input color.

This CL also adds a GM test for SwizzleOutput, since this did not appear
to be covered by existing unit tests.

Change-Id: I3d8176395bb42eab7ff471c9137597402b5b3a69
Bug: skia:10217
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/293884
Commit-Queue: John Stiles <johnstiles@google.com>
Reviewed-by: Greg Daniel <egdaniel@google.com>
3 files changed