halfRenderArea is used by fragment shader to adjust gl_FragCoord for
rotation and yflip compensation. This CL bakes halfRenderAreaWidth into
shader via specialization constant, thus allow compiler to consolidate
the calculation into one MAD instruction.

Bug: b/173800146
Change-Id: Id66301278e3389e2582369b695825e632bccecee
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2551541
Commit-Queue: Charlie Lao <cclao@google.com>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>