Retrieve render colorbuffers as a single vector. 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/215843 Tested-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Brandon Jones <bajones@chromium.org>