[skif] Combine isCropped() and modifiesPixelsBeyondImage()

Change-Id: I9431398a40fc4d8eabaaeda0e5919e27cb10d29b
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/758359
Reviewed-by: Robert Phillips <robertphillips@google.com>
Commit-Queue: Michael Ludwig <michaelludwig@google.com>
3 files changed