Removing the Renderer pointer is one step towards making FBO
attachments a minimal state object. Eventually we will be able
to store them as arrays instead of arrays of pointers.

BUG=angle:660

Change-Id: Idce34e06c339ecb18c60fef12d2ed911d0c4e0f6
Reviewed-on: https://chromium-review.googlesource.com/201835Tested-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Shannon Woods <shannonwoods@chromium.org>