backend: add get_drm_fd to interface

This function allows backends to return the DRM FD they are using. This
will allow the allocator and the renderer to use the right device.

backend/backend.c

@@ -16,6 +16,7 @@
 #include <wlr/backend/wayland.h>
 #include <wlr/config.h>
 #include <wlr/util/log.h>
+#include "backend/backend.h"
 #include "backend/multi.h"
 
 #if WLR_HAS_X11_BACKEND
@@ -71,6 +72,13 @@ clockid_t wlr_backend_get_presentation_clock(struct wlr_backend *backend) {
 	return CLOCK_MONOTONIC;
 }
 
+int backend_get_drm_fd(struct wlr_backend *backend) {
+	if (!backend->impl->get_drm_fd) {
+		return -1;
+	}
+	return backend->impl->get_drm_fd(backend);
+}
+
 static size_t parse_outputs_env(const char *name) {
 	const char *outputs_str = getenv(name);
 	if (outputs_str == NULL) {

include/backend/backend.h

@@ -0,0 +1,8 @@
+#ifndef BACKEND_H
+#define BACKEND_H
+
+#include <wlr/backend.h>
+
+int backend_get_drm_fd(struct wlr_backend *backend);
+
+#endif

include/wlr/backend/interface.h

@@ -19,6 +19,7 @@ struct wlr_backend_impl {
 	struct wlr_renderer *(*get_renderer)(struct wlr_backend *backend);
 	struct wlr_session *(*get_session)(struct wlr_backend *backend);
 	clockid_t (*get_presentation_clock)(struct wlr_backend *backend);
+	int (*get_drm_fd)(struct wlr_backend *backend);
 };
 
 /**