Update GrFragmentProcessor::Rect to use main(float2).
Update GrFragmentProcessor::Rect to use main(float2).

Change-Id: I9d906370827f3f29bf1b321261d8a6bbdc647492
Bug: skia:13737
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/580593
Reviewed-by: Michael Ludwig <cb1f3dd24de0f0b611b26bc28bec758a551d9b7d@google.com>
Auto-Submit: John Stiles <f4fcf42d3bb5924557f1eeb3be66747535e585da@google.com>
Commit-Queue: John Stiles <f4fcf42d3bb5924557f1eeb3be66747535e585da@google.com>

google/skia,google/skia,google/skia,google/skia,google/skia,google/skia,google/skia,google/skia,google/skia,google/skia