[graphite] Improvement to image sampling performance.

* Add flag check before running color transform function

In my testing, drops FS ALU count from 81M to 56M.

Change-Id: I0cc670bdbf1e994adad277b448e390be6410d853
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/779418
Commit-Queue: Jim Van Verth <jvanverth@google.com>
Reviewed-by: James Godfrey-Kittle <jamesgk@google.com>
3 files changed