output: replace wlr_output.damage with a damage event

This patch disambiguates the needs_frame event by uncoupling it from
damage. A new separate damage event is emitted when the backend damages
the output (this happens e.g. VT is changed or software cursors are
used). The event specifies the damaged region.

The wlr_output.damage field is removed. wlr_output is no longer
responsible for tracking its own damage, this is wlr_output_damage's
job.

This is a breaking change, but wlr_output_damage users shouldn't need an
update.

Bugs fixed:

- Screen flashes on VT switch
- Cursor damage issues on the X11 and headless backends

Closes: https://github.com/swaywm/sway/issues/5086

include/wlr/types/wlr_output.h

@@ -138,7 +138,6 @@ struct wlr_output {
 
 	bool needs_frame;
 	// damage for cursors and fullscreen surface, in output-local coordinates
-	pixman_region32_t damage;
 	bool frame_pending;
 	float transform_matrix[9];
 
@@ -150,8 +149,11 @@ struct wlr_output {
 	struct {
 		// Request to render a frame
 		struct wl_signal frame;
-		// Emitted when buffers need to be swapped (because software cursors or
-		// fullscreen damage or because of backend-specific logic)
+		// Emitted when software cursors or backend-specific logic damage the
+		// output
+		struct wl_signal damage; // wlr_output_event_damage
+		// Emitted when a new frame needs to be committed (because of
+		// backend-specific logic)
 		struct wl_signal needs_frame;
 		// Emitted right before commit
 		struct wl_signal precommit; // wlr_output_event_precommit
@@ -181,6 +183,11 @@ struct wlr_output {
 	void *data;
 };
 
+struct wlr_output_event_damage {
+	struct wlr_output *output;
+	pixman_region32_t *damage; // output-buffer-local coordinates
+};
+
 struct wlr_output_event_precommit {
 	struct wlr_output *output;
 	struct timespec *when;

include/wlr/types/wlr_output_damage.h

@@ -52,6 +52,7 @@ struct wlr_output_damage {
 	struct wl_listener output_transform;
 	struct wl_listener output_scale;
 	struct wl_listener output_needs_frame;
+	struct wl_listener output_damage;
 	struct wl_listener output_frame;
 	struct wl_listener output_commit;
 };