Leveraging the fact that there are only a handful of depth/stencil
formats, the last element of RenderPassDesc::mAttachmentFormats can
contain other information in addition to the depth/stencil format ID.

The FormatID enum is rearranged such that depth/stencil formats are
placed first, as a result occupying values in [1, 7].  These values take
up 3 bits in an element of RenderPassDesc::mAttachmentFormats (which
could be the last element if all color draw buffers are attached).

As a result, the upper 5 bits of the last element of
RenderPassDesc::mAttachmentFormats is unused.  It is intended for these
bits to be used in the implementation of multisampled-render-to-texture
depth/stencil renderbuffers.

Bug: angleproject:4836
Change-Id: I0786e0712539cdbbf5494ec83aeee1dd93f936dd
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2413165
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>