Blit11 would clip the destination rectangle with the destination size
but ignore the result. gl::ClipRectangle returns false when the
rectangles do not intersect at all, indicating the blit can be skipped.

This could lead to an out-of-bounds write to the GPU memory for the
destination texture.

Mark ClipRectangle as nodiscard to prevent future issues.

Bug: chromium:1199402
Change-Id: I260e82d0917b8aa7e7887f2c9f7ed4b1a03ba785
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2836786Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Geoff Lang <geofflang@chromium.org>