We were being too permissive in some copyImage methods when checking
if we can create a render target or not. This would lead us to trying
to use an inconsistent TexStorage in some cases.

Note that we do need to create inconsistent TexStorage in cases where
we are doing a FBO copy *from* a mipmap-incomplete texture attachment.

BUG=angle:780

Change-Id: I5a849b5d8c53713e38cb5f5052a8bb88b3f56260
Reviewed-on: https://chromium-review.googlesource.com/224480Tested-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Shannon Woods <shannonwoods@chromium.org>