Rewrite mat2(vec4) casts in GLSL backend to avoid driver bugs.

Context: https://groups.google.com/g/skia-discuss/c/xeD66VLmNN4/m/kNyOW1jGCAAJ

Change-Id: Ic7f337032cc53954df5ec83532059d94fe495b82
Bug: skia:13559, skia:12179
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/561840
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: John Stiles <johnstiles@google.com>
9 files changed