multi-backend: do not expose internal renderers

backend_get_renderer() is now returning the renderer of the primary GPU, instead
of its own renderer, since that's the thing which actually does all of the "real"
rendering

wlr_multi_backend_add() is now adding all subbackends (otherwise only one GPU
is handled).

credits: @ascent12

backend/drm/backend.c

@@ -51,7 +51,12 @@ static void backend_destroy(struct wlr_backend *backend) {
 static struct wlr_renderer *backend_get_renderer(
 		struct wlr_backend *backend) {
 	struct wlr_drm_backend *drm = (struct wlr_drm_backend *)backend;
-	return drm->renderer.wlr_rend;
+
+	if (drm->parent) {
+		return drm->parent->renderer.wlr_rend;
+	} else {
+		return drm->renderer.wlr_rend;
+	}
 }
 
 static struct wlr_backend_impl backend_impl = {

backend/multi/backend.c

@@ -143,7 +143,7 @@ bool wlr_multi_backend_add(struct wlr_backend *_multi,
 	struct wlr_renderer *multi_renderer =
 		multi_backend_get_renderer(&multi->backend);
 	struct wlr_renderer *backend_renderer = wlr_backend_get_renderer(backend);
-	if (multi_renderer != NULL && backend_renderer != NULL) {
+	if (multi_renderer != NULL && backend_renderer != NULL && multi_renderer != backend_renderer) {
 		wlr_log(WLR_ERROR, "Could not add backend: multiple renderers at the "
 			"same time aren't supported");
 		return false;