Move GLSLGeneration definition to SkSL

This is a first step in making GrShaderCaps more general so we can use
it in Graphite.

Change-Id: I7fc874e5df8dba2cef05421840a4019e5499ab59
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/473000
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Jim Van Verth <jvanverth@google.com>
17 files changed