img: fix UAF on Reconfigure by refcounting

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.