Elide sample coords for GrSkSLFP if only used for pass-through sampling

If every use of the coords passed to main was for a call to sample that
was converted to pass-through sampling... We don't actually reference
the sample coords, so we can un-set the flag. This prevents the FP from
requesting an extra set of (unused) coords, which saves a varying
between the vertex and fragment shaders.

Bug: skia:11869
Change-Id: I4e15876031717b8bcf642e742bad8ae26d6bd020
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/411871
Commit-Queue: Brian Osman <brianosman@google.com>
Reviewed-by: Michael Ludwig <michaelludwig@google.com>
5 files changed