Making all our render methods query FBO attachments for rendering
in one place will allow us to easily control the MRT peformance
workaround, and simplify the implementation.

BUG=angle:705

Change-Id: I6c476d45b81228d6ffe8831347443994237e3593
Reviewed-on: https://chromium-review.googlesource.com/215843Tested-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Brandon Jones <bajones@chromium.org>