img: fix UAF on Reconfigure by refcounting

Before this commit, there was a use-after-free bug on Reconfigure:
- theme_finish() destroys lab_imgs for titlebar icons
- For some reason, undecorate() calls _create_buffer() in
  scaled-img-buffer.c, which calls img_render() on a destroyed lab_img.

So in this commit, the lifetime of lab_img is expanded to when the
scaled_img_buffers referencing it are all destroyed. This is achieved by
calling lab_img_copy() when setting a lab_img to scaled_img_buffer and
calling lab_img_destroy() when clearing a lab_img.

Now that scaled_img_buffer.img are always different, lab_img_equal() is
added to compare the content of scaled_img_buffer.img.

src/common/scaled-img-buffer.c

@@ -26,6 +26,7 @@ static void
 _destroy(struct scaled_scene_buffer *scaled_buffer)
 {
 	struct scaled_img_buffer *self = scaled_buffer->data;
+	lab_img_destroy(self->img);
 	free(self);
 }
 
@@ -36,7 +37,7 @@ _equal(struct scaled_scene_buffer *scaled_buffer_a,
 	struct scaled_img_buffer *a = scaled_buffer_a->data;
 	struct scaled_img_buffer *b = scaled_buffer_b->data;
 
-	return a->img == b->img
+	return lab_img_equal(a->img, b->img)
 		&& a->width == b->width
 		&& a->height == b->height
 		&& a->padding == b->padding;
@@ -52,12 +53,13 @@ struct scaled_img_buffer *
 scaled_img_buffer_create(struct wlr_scene_tree *parent, struct lab_img *img,
 	int width, int height, int padding)
 {
+	assert(img);
 	struct scaled_scene_buffer *scaled_buffer = scaled_scene_buffer_create(
 		parent, &impl, &cached_buffers, /* drop_buffer */ true);
 	struct scaled_img_buffer *self = znew(*self);
 	self->scaled_buffer = scaled_buffer;
 	self->scene_buffer = scaled_buffer->scene_buffer;
-	self->img = img;
+	self->img = lab_img_copy(img);
 	self->width = width;
 	self->height = height;
 	self->padding = padding;
@@ -73,7 +75,9 @@ void
 scaled_img_buffer_update(struct scaled_img_buffer *self, struct lab_img *img,
 	int width, int height, int padding)
 {
-	self->img = img;
+	assert(img);
+	lab_img_destroy(self->img);
+	self->img = lab_img_copy(img);
 	self->width = width;
 	self->height = height;
 	self->padding = padding;