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.
- fix that icons for normal/hovered/rounded buttons are not placed
exactly the same position
- fix blurry window button icons in scaled outputs
This commit introduces lab_img and scaled_img_buffer and uses them for
rendering icons in the window titlebar. Now the process of rendering
button icons are split into 2 phases: loading with lab_img_load() and
creating scene-nodes for them with scaled_img_buffer_create(). This
might incur some additional overhead since we no longer preload icon
textures, but the rendering of icon only happens for the first window
as backing buffers are shared and the overhead won't be noticeable.
This commit also simplifies the process of centering icon buffer in the
button, by creating icon buffers in a fixed geometry via
lab_img_render().
