Changes gl::RenderbufferStorage to rx::RenderbufferImpl

BUG=angle:782

Also eliminates Colorbuffer/DepthStencilBuffer and related friends
and adds the RenderbufferD3D implementation of RenderbufferImpl.

This version fixes problems with the prior CL's handling of
render target serial issuance.

Change-Id: I7ddfbec1097723d7ea6ce93a9c034f941349108a
Reviewed-on: https://chromium-review.googlesource.com/227195
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Tested-by: Shannon Woods <shannonwoods@chromium.org>