For read only depth stencil buffers, there is no need to store depth or
stencil value. But we can not use DontCare for storeOp because vulkan
core spec says DontCare indicates data is undefined after this.
VK_QCOM_render_pass_store_ops extension introduces a new store op that
will leave data defined but skip the store. This CL utilize this if the
extension is available.

Bug: angleproject:5055
Change-Id: I104f3d01eb342a2d0cc900f342430e901bde1bff
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2462604
Commit-Queue: Charlie Lao <cclao@google.com>
Reviewed-by: Tim Van Patten <timvp@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>