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>