Add gl::RenderbufferState shared state helper. This shared state will be read-only visible in the RenderbufferImpl class. It mirrors existing structs for Textures, Buffers, and other classes. It allows the implementation class to have a read-only view as to the current GL state of an object. This will be useful to the Vulkan back-end, which would like to know the current Renderbuffer state before having to redefine the storage. If the current parameters match, it might not have to redefine the storage at all. The solution involves passing around the gl::RenderbufferState through various factory methods. Also name the Renderbuffer implementation pointer consistently and make it use std::unique_ptr. Bug: angleproject:2347 Change-Id: Ied6e0358e24e74a7fedbe4aea692edee909b5838 Reviewed-on: https://chromium-review.googlesource.com/922457 Commit-Queue: Jamie Madill <jmadill@chromium.org> Reviewed-by: Geoff Lang <geofflang@chromium.org>