output: Add commit sequence number

This allows synchronisation between different instances of commit/precommit
callbacks.

include/wlr/types/wlr_output.h

@@ -114,6 +114,9 @@ struct wlr_output {
 
 	struct wlr_output_state pending;
 
+	// Commit sequence number. Incremented on each commit, may overflow.
+	uint32_t commit_seq;
+
 	struct {
 		// Request to render a frame
 		struct wl_signal frame;

types/wlr_output.c

@@ -311,6 +311,7 @@ void wlr_output_init(struct wlr_output *output, struct wlr_backend *backend,
 	wl_list_init(&output->modes);
 	output->transform = WL_OUTPUT_TRANSFORM_NORMAL;
 	output->scale = 1;
+	output->commit_seq = 0;
 	wl_list_init(&output->cursors);
 	wl_list_init(&output->resources);
 	wl_signal_init(&output->events.frame);
@@ -500,6 +501,7 @@ bool wlr_output_commit(struct wlr_output *output) {
 	output->needs_frame = false;
 	output_state_clear(&output->pending);
 	pixman_region32_clear(&output->damage);
+	output->commit_seq++;
 	return true;
 }