GrFP can express distance vector field req., program builder declares variable for it

GrFP can express distance vector field req.,  program builder declares variable for it

This update allows fragment processors to require a field of vectors to the nearest edge. This requirement propagates:

- from child FPs to their parent
- from parent FPs to the GrPaint
- from GrPaint through the PipelineBuilder into GrPipeline
- acessed from GrPipeline by GrGLSLProgramBuilder

GrGLSL generates a variable for the distance vector and passes it down to the GeometryProcessor->emitCode() method.

This CL's base is the CL for adding the BevelNormalSource API: https://codereview.chromium.org/2080993002

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2114993002

Committed: https://skia.googlesource.com/skia/+/4ef6dfa7089c092c67b0d5ec34e89c1e319af196
Review-Url: https://codereview.chromium.org/2114993002