Vulkan does not support receiving out of bounds numbers for the scissor
region but no validation errors are sent. This change clips the region
received to fit the current viewport and it fixes
many dEQP tests.

Bug:angleproject:2356
Change-Id: I5dcf739366ba1f62b56593eaaccedf5f14d0c200
Reviewed-on: https://chromium-review.googlesource.com/957758Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Luc Ferron <lucferron@chromium.org>