SkSL does not support shrinking a vector via casting. Use a swizzle
instead.
Change-Id: Ieba78a05dad9c55f44c765924e28f0c7e1667a67
Bug: skia:12193
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/427198
Auto-Submit: John Stiles <johnstiles@google.com>
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
Reviewed-by: Ethan Nicholas <ethannicholas@google.com>
