Fix matrix layout in Metal.

2x2 matrices are packed tightly, 3x3 have a pad of float at the end
of each row.

Bug: skia:8243
Change-Id: I07c3d051643aed13e37bc52a13cdd67a3728d71b
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/214364
Reviewed-by: Greg Daniel <egdaniel@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>
Commit-Queue: Jim Van Verth <jvanverth@google.com>
2 files changed