The VK_QUERY_PIPELINE_STATISTIC_CLIPPING_INVOCATIONS_BIT query produces
the same result as the GL primitives generated query.  One caveat is
that in combination with rasterizer discard this query may not work.
This is emulated by disabling rasterizer discard when this query is
active and applying an empty scissor instead.

When VK_EXT_primitives_generated_query is released and supported, a
similar issue with rasterizer discard persists so this change will
facilitate using that extension as well.

Bug: angleproject:5430
Change-Id: Id45b6f058c5cb6837e04aa64b1efde28c104e4cf
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2976181
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>