In the D3D renderer, changing the base level may trigger re-allocation
of the texture storage, for example if the new base level has a
different aspect ratio. During the process, image contents in the
texture storage should be backed up properly. The D3D11 backend does
this if an image has been associated with the texture storage, but it
may happen such an association has never been established, and
corruption will be observed then.

The proposed patch mitigates the problem by introducing a new method
named findRenderTarget(), with which one can tell if a mip level has
been used as the render target. This works based on the fact that render
targets are cached in the texture storage object. Hence all mip levels
of interest can be found, without relying on the association between
images and texture storage.

Bug: angleproject:2291
Change-Id: Ic73af7b603be25c65760928f276bec16df003baf
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2158830
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>