Eliminate the potential for integer overflow when clippling a large
viewport rectangle by first limiting it to the Vulkan viewport size
limits.

BUG=chromium:1078378

Change-Id: I2648c6136d2d27d67a3fc5dae2de821279d70d81
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2215308Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>
Commit-Queue: Geoff Lang <geofflang@chromium.org>