streamline some of the _matrixProcs

  - We're always tiling the same way in X and Y,
    so fold that through in a few ways.
  - Simplify the interface between _matrixProcs.cpp and _matrix.h.

Cq-Include-Trybots: luci.chromium.try:linux-blink-rel
Change-Id: If306b4506b960bfe8156d0f3f5d28f14ccf4b40a
Reviewed-on: https://skia-review.googlesource.com/c/172023
Reviewed-by: Mike Klein <mtklein@google.com>
Commit-Queue: Ravi Mistry <rmistry@google.com>
3 files changed