Add readonly/writeonly modifiers to SkSL.

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

Change-Id: If811706b3f2ca3bbddda69c094bcc7009f86b31c
Bug: skia:13609
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/565007
Auto-Submit: John Stiles <johnstiles@google.com>
Commit-Queue: John Stiles <johnstiles@google.com>
Reviewed-by: Arman Uguray <armansito@google.com>
16 files changed