pixman-renderer: formalize renderbuffers a bit

This implements the first (pixman) part of @daniels' suggestions in !1106 (comment 1718649).

Add a struct pixman_renderer_interface and hang it off of struct weston_renderer. For starters, this interface is used to access pixman renderer output state creation/removal and to create/destroy struct weston_renderbuffers, which for now just wrap a pixman_image_t with an added damage region for damage tracking. The current renderbuffer is passed as a new parameter to repaint_output instead of setting it with a custom function.