Restrict use of TexStorage more in CopyImage. 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/224480 Tested-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Geoff Lang <geofflang@chromium.org> Reviewed-by: Shannon Woods <shannonwoods@chromium.org>