SkSL: Disallow swizzles that *only* contain '0' or '1' components

These are very easy to mis-read, and they don't make much sense.
Swizzles should refer to the base expression at least once.

Change-Id: I4c2740c7439a11b51b2eb41263c6435e3aefad35
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/310376
Commit-Queue: Brian Osman <brianosman@google.com>
Auto-Submit: Brian Osman <brianosman@google.com>
Reviewed-by: John Stiles <johnstiles@google.com>
3 files changed