Use GrGLGpu::createTextureImpl from w/in GrGLGpu::createBackendTexture

In general we want to have one way to create textures which is shared by the two use cases (i.e., Skia proper and explicit allocation API).
This brings the two paths (i.e., createBackendTexture and onCreateTexture) into closer alignment.

Change-Id: I176a6ebdc6e8634d3aaa59d440b2099df2bd6eff
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/216613
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
3 files changed