Add .stage output to skslc for runtime effects

This emits SkSL that is more-or-less what the compiler re-ingests when a
runtime effect is used to create a GrFragmentProcessor.

Change-Id: I0926be44fc4493e722a5edc18198e161e4192cde
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/367883
Commit-Queue: Brian Osman <brianosman@google.com>
Reviewed-by: John Stiles <johnstiles@google.com>
15 files changed