Since now ImageHelper object has a mRenderPassUsageFlags tracking how it
is been used by current RenderPassCommands, we can consolidate
mReadOnlyDepthStencilMode into a bit in the mRenderPassUsageFlags. The
read only mode is just a special attachment mode, so this makes code
more consistent and able to get rid of the extra dword for
mReadOnlyDepthStencilMode.

Bug: b/181797383
Change-Id: Ie9b7be1c18c392e5b2712bdae6ab6506de8d6d34
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2821942
Commit-Queue: Charlie Lao <cclao@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>