Remove old SkSL JIT and cleanup include files a bit

- The raster pipeline JIT was an experiment that never really panned
  out (except for also introducing the interpreter, which survives).
  The removal should be pretty thorough, including undoing some changes
  to SkRasterPipeline itself. For reference, I basically undid most of
  https://skia-review.googlesource.com/c/skia/+/112204/

- With runtime shaders, always convert to Program using kPipelineStage
  This makes the first-pass compile consistent with runtime color filter,
  and with GrSkSLFP.

- Remove sksl_mixer.inc, which hasn't been used in a while.

- In sksl_pipeline.inc, all of the math symbols are already declared
  in sksl_gpu.inc, which forms the base symbol table when this include
  is parsed.

  sk_x and sk_y appear to never be used? Those builtin IDs are the IDs
  of the x and y params to main, but that logic still works without
  these declarations.

  sk_OutColor only makes sense in FP files (it's still declared in
  sksl_fp.inc).

Change-Id: Ie8dd68d2c4687745d46f96804a76695bce8c1ba2
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/246017
Reviewed-by: Ethan Nicholas <ethannicholas@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
20 files changed