Enable outerProduct() in public SkSL and add unit tests.

Compile-time optimization is not yet implemented so the generated code
contains a lot of checks which will be optimized away in a followup CL.

Change-Id: I83b5df8580a6712686d18812e3848a703feac315
Bug: skia:12202
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/447300
Commit-Queue: John Stiles <johnstiles@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
7 files changed