backends: implement custom EGL and renderer initialization

Compositors now have more control over how the backend creates its
renderer. Currently all backends create an EGL/GLES2 renderer, so
the necessary attributes for creating the context are passed to a
user-provided callback function. It is responsible for initializing
provided wlr_egl and to return a renderer. On fail, return 0.

Fixes #987

examples/rotation.c

@@ -235,7 +235,7 @@ int main(int argc, char *argv[]) {
 	};
 	wl_list_init(&state.outputs);
 
-	struct wlr_backend *wlr = wlr_backend_autocreate(display);
+	struct wlr_backend *wlr = wlr_backend_autocreate(display, NULL);
 	if (!wlr) {
 		exit(1);
 	}