Some Vulkan drivers are not clipping rendering that is outside of the viewport,
and the Vulkan spec has some language that indicates that ANGLE should set the
scissor:

  The application must ensure (using scissor if necessary) that all rendering
  is contained within the render area.

In this case, ANGLE is "the application".

Bug: angleproject:3253
Bug: angleproject:3254
Change-Id: I6885a3aa6daed57f6ae1c3d974663d763cad10c9
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1553973Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Ian Elliott <ianelliott@google.com>