Always generate the same pixel shader no matter what framebuffer is bound.

BUG=angle:670

Change-Id: I3fa91ff43363d49b38a19d3815ea523946c675d5
Reviewed-on: https://chromium-review.googlesource.com/204573Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Geoff Lang <geofflang@chromium.org>