Allow SkImage pointer in GetBackendTextureFromImage

While Skia exclusively uses the sk_sp version, clients seem to use
a mix of sk_sp and raw pointers. It seems silly to require clients
to wrap the pointer just to have it be unwrapped, so this extra alias
should make the transition more seamless.

Change-Id: Iaa45f9e75848297386c8cf261c69c08fddd99dac
Bug: skia:13983
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/666857
Reviewed-by: Brian Osman <brianosman@google.com>
3 files changed