We have a region for the border here, but the color is defined as
transparent black because we don't want to render anything for it.
This way, the generated .node file for the listbox demo in gtk4-demo
with enabled layout borders is 3.5MB instead of 3.8MB.
Replace uses of gtk_css_style_get_value with direct access,
throughout the tree. We don't replace all uses, just those
where we are dealing with a fixed property. Be careful to
handle the currentColor special case for color properties.
This code is better because:
1) The coordinates translations are actually correct and not sometimes
wrong like before
2) We clip widgets that have overflow set
3) We honor the widget's transform
4) It is less code
