Unless the RenderTarget9 is backed by a texture, it's not possible to
sample from it.  This is required to sample from D3D9 renderbuffers when
they are bound to texture targets.

BUG=angleproject:970

Change-Id: Id667a5e44c1f959609bc653a974ede5b93ec9a3e
Reviewed-on: https://chromium-review.googlesource.com/293290Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Geoff Lang <geofflang@chromium.org>