Add integer texture support.

This allows us to create integer textures and sample them from a GrProcessor's code.

Filtering is limited to NEAREST.

Adds tests for reading/writing pixels, copying, and drawing. These operations are not allowed to convert to fixed/float configs.

Vulkan support is TBD.


GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=4348

Change-Id: If38d89a03285d4bd98d1f14f9638b0320977e43d
Reviewed-on: https://skia-review.googlesource.com/4348
Commit-Queue: Brian Salomon <bsalomon@google.com>
Reviewed-by: Chris Dalton <csmartdalton@google.com>