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/multi-pointer.c

@@ -284,7 +284,7 @@ int main(int argc, char *argv[]) {
 		.clear_color = { 0.25f, 0.25f, 0.25f, 1 },
 		.display = display,
 	};
-	struct wlr_backend *wlr = wlr_backend_autocreate(display);
+	struct wlr_backend *wlr = wlr_backend_autocreate(display, NULL);
 	if (!wlr) {
 		exit(1);
 	}