EXT_multisampled_render_buffer2 specifies that depth/stencil textures
are expected to be in a perpetual state of invalidated, except during
rendering.  This change makes sure that they never use loadOp=LOAD.

Additionally fixes a bug where clears applied to MSRTT depth/stencil
textures didn't take effect because they were applied to the
multisampled image (since the resolved image was not given to the render
target).

Bug: angleproject:4836
Bug: angleproject:5063
Change-Id: I4506f4de415dca6c222111a1ae62017d2fb1e2b1
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2412848
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>