If a texture is simultaneously attached to two FBOs, one where it's a
normal texture and another where it's multisampled-render-to-texture,
different render targets must be created for it.

If a texture is simultaneously attached to two FBOs, both as
multisampled-render-to-texture but with different sample counts, two
implicit multisampled images need to be created as well as different
render targets.

Bug: angleproject:4913
Change-Id: I584ba327e4cb2099ef62f86f5d88719dc156ce13
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2335810
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>