drm backend: add wlr_output_is_drm

backend/drm/drm.c

@@ -658,6 +658,10 @@ static struct wlr_output_impl output_impl = {
 	.get_gamma_size = wlr_drm_connector_get_gamma_size,
 };
 
+bool wlr_output_is_drm(struct wlr_output *output) {
+	return output->impl == &output_impl;
+}
+
 static int retry_pageflip(void *data) {
 	struct wlr_drm_connector *conn = data;
 	wlr_log(L_INFO, "%s: Retrying pageflip", conn->output.name);

include/backend/drm/drm.h

@@ -12,7 +12,6 @@
 
 #include <wlr/backend/session.h>
 #include <wlr/backend/drm.h>
-#include <wlr/types/wlr_output.h>
 #include <wlr/render/egl.h>
 
 #include "iface.h"

include/wlr/backend/drm.h

@@ -4,10 +4,12 @@
 #include <wayland-server.h>
 #include <wlr/backend/session.h>
 #include <wlr/backend.h>
+#include <wlr/types/wlr_output.h>
 
 struct wlr_backend *wlr_drm_backend_create(struct wl_display *display,
 	struct wlr_session *session, int gpu_fd, struct wlr_backend *parent);
 
 bool wlr_backend_is_drm(struct wlr_backend *backend);
+bool wlr_output_is_drm(struct wlr_output *output);
 
 #endif