Make GrGLGpu::createBackendTexture parallel GrGLGpu::createTextureImpl

- in preparation for making createBackendTexture call createTextureImpl (just like GrGLGpu::onCreateTexture)

This is more pre-work for making https://skia-review.googlesource.com/c/skia/+/214445/ (Expand backend allocation API to allow an initialization color) reasonable to review.

Change-Id: I28a9ec889532fb72fd05bea4b1fdbb8935e5228b
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/215170
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
3 files changed