Allow readonly and writeonly as function-params on textures.

Change-Id: I925e3bc932a86c999897ccb2b80e3977864c1b13
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/565010
Auto-Submit: John Stiles <johnstiles@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: John Stiles <johnstiles@google.com>
6 files changed