Writing to all 8 pixel outputs was causing performance problems on
Intel and AMD. Enabling Geoff's work to rewrite our pixel shaders
solves the regression.

This patch also includes a workaround to the nVidia driver bug
where it would ignore NULL RT values in OMSetRenderTargets, by
compacting the RT list to skip NULL values.

BUG=angle:705
BUG=365078

Change-Id: Ia68af6f0ccd5f10c484d6f76297a0bec694948f0
Reviewed-on: https://chromium-review.googlesource.com/214852Tested-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>