Used angle::CheckedNumeric to make sure none of the additions
in ClipRectangle produces an integer overflow.

Bug: chromium:1091364
Change-Id: I23b7943259010bb6bfde3d8ebc41e87a04f5b4f6
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2404441Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Alexis Hétu <sugoi@chromium.org>