Fix non-zero special-image subsets in magnifier filter

This will let us re-land https://skia-review.googlesource.com/c/skia/+/237127
which makes non-zero subsets much more common. Confirmed that this CL and
the linked CL will now pass the CC unittests that caught the original failure.

Change-Id: Ic0393a574b2c19d92421a5deece81ab2ed70fc97
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/237899
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Michael Ludwig <michaelludwig@google.com>
3 files changed