Cleanup of GrSkSLFP

- Move child processor function generation into a separate utility
- No need to pass coords to expandFormatArgs, just use args.fSampleCoord
- Rather than a two-character escape sequence for format args (that
  requires special handling of '%'), use a single character that's never
  legal elsewhere. Simplifies the arg expansion logic, and removes the
  need to special-case operator% in the generator.
- Change the type of GLSLFunction::fBody to avoid round-trip casting

Change-Id: Id242b86ab6ee14b8c92c81c1f062810034e71a96
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/299078
Reviewed-by: Michael Ludwig <michaelludwig@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
6 files changed