Detach pipeline-stage generator from SkSL compiler

This is now structured like the VM generator: Just a function that does
the conversion. Moved all relevant types and constants out of the
compiler, too. The key thing is that we don't need/want an error
handler, because it's too late to fail. We *must* catch all errors
during IR generation.

This is also another step along the path of directly emitting to the
fragment shader builder, rather than generating strings with
placeholders.

Bug: skia:11127
Change-Id: I18591270aa6e56dae1f040275a4b7d4a245007db
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/366956
Commit-Queue: Brian Osman <brianosman@google.com>
Reviewed-by: John Stiles <johnstiles@google.com>
9 files changed