Add buffer modifier to SkSL.

These are currently parsed and added to the Modifiers, but not allowed
anywhere by the language.

Change-Id: Ie7f1e451cf880bdc02b892e8ac84cd93fb8e1220
Bug: b/238620644
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/565339
Reviewed-by: James Godfrey-Kittle <jamesgk@google.com>
Commit-Queue: John Stiles <johnstiles@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>
16 files changed