Retract GrTexture.h a bit

This began as cleaning up SkSpecialImage.h & spiraled out of control from there.

Change-Id: I9a570ecd2a7af9ee724ebfebe4e9185748f38bbc
Reviewed-on: https://skia-review.googlesource.com/19290
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
20 files changed