Reland "Fixed some GrSkSLFP limitations."

This is a reland of 190157bff2400d68e8768200569bd9e5000c8338

Original change's description:
> Fixed some GrSkSLFP limitations.
> 
> This makes it possible to pass more types as uniforms / inputs and fixes an
> issue with non-main functions calling other non-main functions.
> 
> Change-Id: I6b5623b3c967d8219b992a455cc68bb0a29706b8
> Reviewed-on: https://skia-review.googlesource.com/c/skia/+/255300
> Reviewed-by: Brian Osman <brianosman@google.com>
> Commit-Queue: Ethan Nicholas <ethannicholas@google.com>

Change-Id: Icc06cd54abb7acc83415915e16ca64c3eb6b943b
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/255779
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
4 files changed