Rename GrPathStencilFillOp -> GrPathStencilCoverOp

"Fill" is an overloaded term. Switch to saying "cover", which I
believe is more true to the Redbook terminology anyway.

Bug: skia:10419
Change-Id: I906122c58462ac374b29dcb96e35bb3f86e55b0d
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/416079
Commit-Queue: Chris Dalton <csmartdalton@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>
6 files changed