Add WGSL support for matrix-op-scalar math.

Like Metal, WGSL only supports matrix-op-scalar natively for
multiplication, so we need to rewrite these expressions into
matrix-op-matrix by splatting the scalar across every matrix
element.

This fixes one test. I've also adjusted the test slightly to
increase its coverage a bit. Previously it only tested
assignment-expressions (`matrix op= scalar`) but neglected
to test `scalar-op-matrix` or `matrix = matrix op scalar`.
I've rewritten the expressions to cover these forms as well.

Change-Id: I41dd4b6fc1ef93a6bbc2b5ed6424147500fdeb9b
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/737056
Auto-Submit: John Stiles <johnstiles@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
9 files changed