added constant 0 and 1 swizzle components to SkSL

Bug: skia:
Change-Id: I8f4dce314ebe0d5de7ba032a435f362eeb14edc4
Reviewed-on: https://skia-review.googlesource.com/c/191565
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
8 files changed