Make it easier to test rectangle textures by using createBackendTexture.

Also allows internal creation of rectangle textures, only used by unit
tests currently.

Previously GrContext::createBackendTexture() would ignore the request
for RECTANGLE or EXTERNAL and always make 2D. Now it makes RECTANGLE if
supported and always fails for EXTERNAL.

Change-Id: Iafbb3f5acddb37bfb8d39740f2590177a07dae78
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/297472
Commit-Queue: Brian Salomon <bsalomon@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>
9 files changed