src mode fast path for full coverage

This doesn't really show up in existing benches,
but this lets src modes bypass loading the destination
entirely when coverage is full.

Change-Id: I3c533a871bc370b42b3325f0dc0abce84798729b
Reviewed-on: https://skia-review.googlesource.com/c/193682
Reviewed-by: Mike Klein <mtklein@google.com>
3 files changed