output-damage: set needs swap only when we have new damage

An output needs swap when there is new damage. If the damage is only
accumulated from the last frame, we could simply skip drawing.

types/wlr_output_damage.c

@@ -144,6 +144,8 @@ bool wlr_output_damage_attach_render(struct wlr_output_damage *output_damage,
 		return false;
 	}
 
+	*needs_frame =
+		output->needs_frame || pixman_region32_not_empty(&output_damage->current);
 	// Check if we can use damage tracking
 	if (buffer_age <= 0 || buffer_age - 1 > WLR_OUTPUT_DAMAGE_PREVIOUS_LEN) {
 		int width, height;
@@ -151,6 +153,7 @@ bool wlr_output_damage_attach_render(struct wlr_output_damage *output_damage,
 
 		// Buffer new or too old, damage the whole output
 		pixman_region32_union_rect(damage, damage, 0, 0, width, height);
+		*needs_frame = true;
 	} else {
 		pixman_region32_copy(damage, &output_damage->current);
 
@@ -170,7 +173,6 @@ bool wlr_output_damage_attach_render(struct wlr_output_damage *output_damage,
 		}
 	}
 
-	*needs_frame = output->needs_frame || pixman_region32_not_empty(damage);
 	return true;
 }