Initial OpenGL Image support.

Initial OpenGL Image support.

This change along with recently landed changes is enough to make the new unit test work and not much else. imageLoad is support but not stores or any other image functions (atomics). Barriers in the shading language or the GL API are not yet hooked up.

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=4182

Change-Id: I5958b7c89e40ae5ee05f7bbaca3b3738162fe5ce
Reviewed-on: https://skia-review.googlesource.com/4182
Reviewed-by: Greg Daniel <egdaniel@google.com>
Reviewed-by: Chris Dalton <csmartdalton@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>