Fixed sample_matrix_constant GM

This GM was not actually doing what the name implied, and when
switched over to use a constant matrix, turned out to not work.

This fixes it so constant matrices properly affect child processors.

Change-Id: I1f6b271dbf43c18515c0c72701bf8bbf60eb4c59
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/293716
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
4 files changed