Add readonly/writeonly modifier support to texture declarations.

`in` and `out` are no longer allowed on opaque-typed globals like
textures or samplers. `readonly` and `writeonly` are now allowed on
texture-type globals (only) instead.

Change-Id: If27cb0e1b9125bebe9417ae8aa392e68f67c66aa
Bug: skia:13609
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/565012
Reviewed-by: Brian Osman <brianosman@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>
Commit-Queue: John Stiles <johnstiles@google.com>
9 files changed