wlr_scene: Be resilient against overflow conditions

If the area calculations for output overlap overflow a signed int, we
may not consider it to be a primary output. Turn this into an unsigned
type so this happens less frequently.
Additionally, it is possible the overflow would produce 0, we can handle
this by simply changing the comparison to more than or equal.

While we're here, let's assert that we always assign a primary output
if there are any intersecting outputs.

types/scene/wlr_scene.c

@@ -296,7 +296,7 @@ static void update_node_update_outputs(struct wlr_scene_node *node,
 
 	struct wlr_scene_buffer *scene_buffer = wlr_scene_buffer_from_node(node);
 
-	int largest_overlap = 0;
+	uint32_t largest_overlap = 0;
 	scene_buffer->primary_output = NULL;
 
 	uint64_t active_outputs = 0;
@@ -326,8 +326,8 @@ static void update_node_update_outputs(struct wlr_scene_node *node,
 			output_box.x, output_box.y, output_box.width, output_box.height);
 
 		if (pixman_region32_not_empty(&intersection)) {
-			int overlap = region_area(&intersection);
-			if (overlap > largest_overlap) {
+			uint32_t overlap = region_area(&intersection);
+			if (overlap >= largest_overlap) {
 				largest_overlap = overlap;
 				scene_buffer->primary_output = scene_output;
 			}
@@ -352,6 +352,10 @@ static void update_node_update_outputs(struct wlr_scene_node *node,
 			wl_signal_emit_mutable(&scene_buffer->events.output_leave, scene_output);
 		}
 	}
+
+	// if there are active outputs on this node, we should always have a primary
+	// output
+	assert(!scene_buffer->active_outputs || scene_buffer->primary_output);
 }
 
 static bool scene_node_update_iterator(struct wlr_scene_node *node,