SkSL: In the front-end, rename PipelineStage to RuntimeEffect

This maps to usage better, and makes some code simpler to understand.
Note that there is still a PipelineStage *back-end*, which is specific
to the runtime-effect FP. A kRuntimeEffect_Kind program can be used to
generate a PipelineStage (for the GPU backend), or an skvm program (for
the CPU backend).

Change-Id: Id3f535db93a239726c595225aafe9467f0d19817
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/344969
Reviewed-by: John Stiles <johnstiles@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
12 files changed