output: switch swap_buffers damage to output-buffer-local coords

This is one more step towards [1]. This gives more freedom to the compositor
wrt. how it handles damage.

[1]: https://github.com/swaywm/wlroots/issues/1363

include/wlr/types/wlr_output.h

@@ -121,7 +121,7 @@ struct wlr_output {
 struct wlr_output_event_swap_buffers {
 	struct wlr_output *output;
 	struct timespec *when;
-	pixman_region32_t *damage;
+	pixman_region32_t *damage; // output-buffer-local coordinates
 };
 
 enum wlr_output_present_flag {
@@ -203,6 +203,9 @@ bool wlr_output_preferred_read_format(struct wlr_output *output,
  * NULL. If the compositor doesn't support damage tracking, set `damage` to
  * NULL.
  *
+ * Damage is given in output-buffer-local coordinates (ie. scaled and
+ * transformed).
+ *
  * Swapping buffers schedules a `frame` event.
  */
 bool wlr_output_swap_buffers(struct wlr_output *output, struct timespec *when,

rootston/output.c

@@ -522,12 +522,17 @@ renderer_end:
 	wlr_renderer_scissor(renderer, NULL);
 	wlr_renderer_end(renderer);
 
-	if (server->config->debug_damage_tracking) {
 	int width, height;
 	wlr_output_transformed_resolution(wlr_output, &width, &height);
+
+	if (server->config->debug_damage_tracking) {
 		pixman_region32_union_rect(&damage, &damage, 0, 0, width, height);
 	}
 
+	enum wl_output_transform transform =
+		wlr_output_transform_invert(wlr_output->transform);
+	wlr_region_transform(&damage, &damage, transform, width, height);
+
 	if (!wlr_output_damage_swap_buffers(output->damage, &now, &damage)) {
 		goto damage_finish;
 	}

types/wlr_output.c

@@ -365,9 +365,6 @@ bool wlr_output_swap_buffers(struct wlr_output *output, struct timespec *when,
 		output->idle_frame = NULL;
 	}
 
-	int width, height;
-	wlr_output_transformed_resolution(output, &width, &height);
-
 	struct timespec now;
 	if (when == NULL) {
 		clock_gettime(CLOCK_MONOTONIC, &now);
@@ -384,19 +381,12 @@ bool wlr_output_swap_buffers(struct wlr_output *output, struct timespec *when,
 	pixman_region32_t render_damage;
 	pixman_region32_init(&render_damage);
 	pixman_region32_union_rect(&render_damage, &render_damage, 0, 0,
-		width, height);
+		output->width, output->height);
 	if (damage != NULL) {
 		// Damage tracking supported
 		pixman_region32_intersect(&render_damage, &render_damage, damage);
 	}
 
-	// Transform damage into renderer coordinates, ie. upside down
-	// TODO: take transformed coords, make the renderer flip the damage
-	enum wl_output_transform transform =
-		wlr_output_transform_invert(output->transform);
-	wlr_region_transform(&render_damage, &render_damage, transform,
-		width, height);
-
 	if (!output->impl->swap_buffers(output, damage ? &render_damage : NULL)) {
 		pixman_region32_fini(&render_damage);
 		return false;