PipelineStageCodeGenerator no longer derives from GLSL generator

This is largely mechanical, with large chunks of the GLSL generator
functionality removed. There are some obvious further improvements to be
made, but it should be easier starting from here.

Change-Id: I93b1aa1936c4151af83f83e6d03233d89682496d
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/366402
Reviewed-by: John Stiles <johnstiles@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
2 files changed