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.
Use the new gtk_css_dimension_value_is_zero() to check if we really need
to e.g. apply a border radius at all.
We compute css boxes a lot so this makes sense here, it especially shows
up during pick(), where we need the border box.
Previously, those numbers stored the values relative to the margin box
of the widget. Now they store values relative to the content box,
thereby getting rid of the last remains of weird coordinate systems.
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.
