Add support for constant folding of matrix-times-matrix.

This code should be easily adaptable to matrix-times-vector as well;
just treat the vector as a 1-row or 1-column matrix. I haven't gotten
around to writing tests for this, though.

Change-Id: If59ae52cd12952b44d3574d54398b2dc66edbcc8
Bug: skia:12819
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/505221
Auto-Submit: John Stiles <johnstiles@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
Reviewed-by: Ethan Nicholas <ethannicholas@google.com>
Commit-Queue: John Stiles <johnstiles@google.com>
3 files changed