We are running out of space in GraphicsPipelineDesc. According to
gpuinfo, the max reported viewport size is 32768. This CL changes
scissor's x/y/w/h from uint32_t to uint16_t, which saves two uint32_t
for other usages.

Bug: b/173800146
Change-Id: Icf2d8ba8ea8a8c412ecef2059401a8d831c410e4
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2557218
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>