Invalidate was previously affecting only the storeOp of the color and
depth/stencil attachments.  With multisampled-render-to-texture
attachments, the storeOp of the resolve attachments were not being
affected.

This change implements the latter, attempting to remove the attachment
altogether if possible.  With MSRTT depth/stencil buffers, this makes
possible the ability to never write depth/stencil data to memory.

Bug: angleproject:4836
Change-Id: I53599e2f4ed6c390dfd03bf226274f6f53f438bb
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2437506
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>