cssboxes: Introduce

cssboxes: Introduce

Split out the code for computing CSS boxes from given variables from the
background render code. This way, it can be shared between different
codebases.

Also, make that code completely be contained of static inline functions.
That ensures that it can be 100% inlined in cases where only parts of
the rectangle are needed (like in gtk_widget_get_width() in the future).

This will require some more patches to actually work, but those will
follow.