Flip the arguments to sample() when passing both coords and a color

A subset of these signatures will be available to public SkSL, and
structured in a way that really pushes the coords as the primary
argument (and color as an optional one). In any case, I find this
ordering more natural.

Change-Id: I7b3bc962c5b305b9eeed1ae11ae1dc2ce7269364
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/396021
Reviewed-by: Michael Ludwig <michaelludwig@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
5 files changed