Add image functions to GrGLInterface

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=4840

Change-Id: I250cc4e637765c321c90e33c9b3f25c4ad12fe04
Reviewed-on: https://skia-review.googlesource.com/4840
Reviewed-by: Chris Dalton <csmartdalton@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>
5 files changed