When minimize-gpu-work is requested, the idea is to have GPU work
reduced to minimum. Without shrink the offscreen window to 1x1, the
driver load/store op may still have to load entire surface which could
be costly. This CL reduces the offscreen surface size to 1x1 when
--minimize-gpu-work is specified.

Bug: b/184766477
Change-Id: Ic8d9d4c2fe8295a0a5808dcee35120fdd20e0f56
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2840963
Commit-Queue: Charlie Lao <cclao@google.com>
Reviewed-by: Cody Northrop <cnorthrop@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>