Add set*Attachment methods to FramebufferImpl.

BUG=angle:841

Change-Id: Ie819c253a900d105d768aee168a6a2de89754ccc
Reviewed-on: https://chromium-review.googlesource.com/232393
Reviewed-by: Brandon Jones <bajones@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Geoff Lang <geofflang@chromium.org>
Rename Framebuffer::completeness to checkStatus and add an Impl method.

BUG=angle:841

Change-Id: I04b4ffd086424569a15aa21447dd552e0a898928
Reviewed-on: https://chromium-review.googlesource.com/232394
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Tested-by: Geoff Lang <geofflang@chromium.org>