This texture parameter describes a cropping region x,y, width, height.

Bug: angleproject:6057
Change-Id: I2c71f6aec1e536f42e06c19cb025889094b57a35
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2956510Reviewed-by: Cody Northrop <cnorthrop@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>