GLSL supports casting vec4 into mat2 and vice versa, so SkSL should have
equivalent support. Adding tests as a starting point.
Change-Id: If8bcbf99afcec94d948d5da9e6205cb4a232af18
Bug: skia:12067
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/425837
Auto-Submit: John Stiles <johnstiles@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
