If glFramebufferSubInvalidate() is called with an area that covers the
whole framebuffer, behave as if glFramebufferInvalidate() is called.
This allows deferred clears to be removed for example, and attachment
contents to be marked undefined.

Bug: angleproject:4988
Change-Id: Iff3f291ea6c07abccc2740174d0451b432ac5da8
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2508977Reviewed-by: Charlie Lao <cclao@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>