Rename DestColor FP to SurfaceColor.

This is meant to help disambiguate the FP from `args.fDestColor`.

Change-Id: I59a4527e64970811a6798a635191016c645587ce
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/434679
Commit-Queue: John Stiles <johnstiles@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>
5 files changed