Multiview is supported in Vulkan simply by specifying the number of
views in the render pass, and creating the appropriate image views.  A
number of changes to the way image views and render targets are stored
are made to support those that don't cover the entire range of layers.

One particular detail that is not implemented in this change is the use
of queries in combination with multiview.  Vulkan specifies that N
queries are actually produced (N being the number of views) which must
be summed by the application, but this is not currently done.

Bug: angleproject:6048
Change-Id: I1d4a9894c232d3a93d7a97c9fa0eedc334e57469
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2967625
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Charlie Lao <cclao@google.com>