Implement computeFastBounds in SkLocalMatrixImageFilter

This also adds a GM based on Jim's fiddle: https://fiddle.skia.org/c/781234e35c208ee03f79b90613117b91
I confirmed that it never flickers when animating in viewer.

Patchset 1 shows the GM being blank (with the computeFastBounds fix
disabled). Patchset 3 shows that with the fix enabled, the GM is not
blank.

Bug: skia:9282
Change-Id: I206f7150c395b0a35ecf0455e4905f72ae057e6b
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/295558
Commit-Queue: Michael Ludwig <michaelludwig@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>
3 files changed