Replace rx::Renderer with rx::ContextImpl. Previously Context had no Impl class, but had a special relationship with the instanced Renderer class. Having a ContextImpl backing every Context will allow new designs to enable things like multithreading (where each ContextImpl stores a Context-specific device) or non- virtual Contexts on Android or other platforms where it is more efficient. A large refactoring patch that touches every back-end. BUG=angleproject:1363 Change-Id: Icb73a7d37447f08a664eeb499a310ba05d71a57e Reviewed-on: https://chromium-review.googlesource.com/342052 Reviewed-by: Corentin Wallez <cwallez@chromium.org> Commit-Queue: Jamie Madill <jmadill@chromium.org>