Add backend detection functions

Resolves #80

backend/drm/backend.c

@@ -52,6 +52,10 @@ static struct wlr_backend_impl backend_impl = {
 	.get_egl = wlr_drm_backend_get_egl
 };
 
+bool wlr_backend_is_drm(struct wlr_backend *b) {
+	return b->impl == &backend_impl;
+}
+
 static void session_signal(struct wl_listener *listener, void *data) {
 	struct wlr_drm_backend *backend =
 		wl_container_of(listener, backend, session_signal);

backend/libinput/backend.c

@@ -103,6 +103,10 @@ static struct wlr_backend_impl backend_impl = {
 	.destroy = wlr_libinput_backend_destroy
 };
 
+bool wlr_backend_is_libinput(struct wlr_backend *b) {
+	return b->impl == &backend_impl;
+}
+
 static void session_signal(struct wl_listener *listener, void *data) {
 	struct wlr_libinput_backend *backend = wl_container_of(listener, backend, session_signal);
 	struct wlr_session *session = data;

backend/multi/backend.c

@@ -1,3 +1,4 @@
+#include <assert.h>
 #include <stdbool.h>
 #include <stdlib.h>
 #include <wlr/backend/interface.h>
@@ -111,6 +112,8 @@ static void output_remove_reemit(struct wl_listener *listener, void *data) {
 
 void wlr_multi_backend_add(struct wlr_backend *_multi,
 		struct wlr_backend *backend) {
+	assert(wlr_backend_is_multi(backend));
+
 	struct wlr_multi_backend *multi = (struct wlr_multi_backend *)_multi;
 	struct subbackend_state *sub = calloc(1, sizeof(struct subbackend_state));
 	sub->backend = backend;
@@ -135,10 +138,8 @@ void wlr_multi_backend_add(struct wlr_backend *_multi,
 }
 
 struct wlr_session *wlr_multi_get_session(struct wlr_backend *_backend) {
-	// TODO: assert(wlr_backend_is_multi(_backend));
+	assert(wlr_backend_is_multi(_backend));
-	if (_backend->impl != &backend_impl) {
+
-		return NULL;
-	}
 	struct wlr_multi_backend *backend = (struct wlr_multi_backend *)_backend;
 	return backend->session;
 }

include/wlr/backend/drm.h

@@ -9,4 +9,6 @@
 struct wlr_backend *wlr_drm_backend_create(struct wl_display *display,
 		struct wlr_session *session, struct wlr_udev *udev, int gpu_fd);
 
+bool wlr_backend_is_drm(struct wlr_backend *backend);
+
 #endif

include/wlr/backend/libinput.h

@@ -12,4 +12,6 @@ struct wlr_backend *wlr_libinput_backend_create(struct wl_display *display,
 		struct wlr_session *session, struct wlr_udev *udev);
 struct libinput_device *wlr_libinput_get_device_handle(struct wlr_input_device *dev);
 
+bool wlr_backend_is_libinput(struct wlr_backend *backend);
+
 #endif