[graphite] Modify MatrixConvolutionImageFilter's SkSL generation

Bug: b/238759147
Change-Id: I86aa12020a731c3d092bf1469268cc935f450875
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/848223
Commit-Queue: Robert Phillips <robertphillips@google.com>
Reviewed-by: Michael Ludwig <michaelludwig@google.com>
1 file changed