Have gpu createTestingOlyBackendTexture know if it is a render target or not

In Vulkan we need to know at create time of a texture whether or not we will
use it as a render target.

BUG=skia:5458
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2093943002

Review-Url: https://codereview.chromium.org/2093943002
8 files changed