Instead of going through a big switch of all properties,
make the new_compute() functions for each values struct
directly set its members. This way, we can also avoid the
border-width special case for more properties.

Verify that we don't miss properties or have them
more than once.

This was showing up as reftest failures,
sporadically

These are from the GVariant days of render nodes.

It's actually implemented in gtkcssdimensionvalue.c these days.

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.

We can usually omit computations when the dimension value ends up being
zero anyway.

Don't even invert transforms if we can do it easily ourselves. This also
fixes a small GtkTransform memory leak.