Support sample(matrix) in SkRuntimeEffect's CPU backend

Also support the "N" variant of several instructions (for doing
component-wise math on matrices). With these changes, all cases
in sksl_sample_chaining work on the CPU backend, too.

Change-Id: Ie439153da452c80127e314e48fa0fa751d9f8d33
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/299378
Reviewed-by: Mike Klein <mtklein@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
3 files changed