Use PipelineStageArgs throughout GrSkSLFP and PipelineStageCodeGenerator

Making it easier to modify the objects that are emitted by the generator
and consumed by the FP (as a pre-cursor to sampling children with
explicit coords).

Change-Id: Id746a23537c97ba7a8ac27a63622503f59c902f5
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/266377
Reviewed-by: Ethan Nicholas <ethannicholas@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
4 files changed