Design rendering interface for C backend

[sdilts]

There needs to be some kind of interface in the C backend that enables the lisp code to tell it what to render. The two things we should focus on supporting right now is rendering surfaces via wlroots' scene graph API and drawing window borders for focused windows / frames.

[sdilts]

The scene graph API has the ability to render wlr_buffers and normal rectangles, so rendering borders can just be done as part of the scene graph manipulation. Although it's more complex than we need right now, Sway's (yet to be merged) usage of this API should give us some hints.