[canvaskit] Properly downsample 4x4 matrices.

Also address feedback from Tuesday.

Change-Id: I4497e4112c758a071c834045d20a009397184254
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/356837
Reviewed-by: Nathaniel Nifong <nifong@google.com>
4 files changed