Switch coord transforms in gradient layouts to 3x3 (SkMatrix)

These were using 4x4 (SkMatrix44), but only temporarily, and it failed
with an upcoming change that makes SkMatrix44 -> SkMatrix explicit.

Change-Id: I8aa9adb209f10d2c04ca009d6d5f7fdafc264ae9
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/264998
Reviewed-by: Michael Ludwig <michaelludwig@google.com>
Reviewed-by: Ethan Nicholas <ethannicholas@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
8 files changed