Instead of always switching the Framebuffer to mask out depth/stencil
loops we now switch the RenderPass to a "read-only" depth/stencil mode.

Reduces the RenderPass count in Manhattan from 18->15. There are still
a couple extra RenderPasses inserted that we can get rid. We can merge
a few RenderPasses by retroactively changing a started RenderPass to
"read-only" when there are no prior recorded depth writes or clears.

Also adds a test to count the number of RenderPasses ANGLE generates in
DS feedback loop situaions.

Bug: angleproject:4959
Change-Id: I1855a45959655fc27ccd47a3469c1c672fc8fd9e
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2357973
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Charlie Lao <cclao@google.com>
Reviewed-by: Tim Van Patten <timvp@google.com>