It's using a GtkCssPositionValue, even though that name is wrong. But
the functionality of managing 2 lengths is exactly what we want.
Nobody is using this yet.
Parsing a shorthand background property was running into unexpected
errors when trying position values where there were none. To fix this,
introduce a try_parse variant of the position parse function that
silently returns NULL.
This commit is essentially a large reorganization. Instead of all value
subtypes having their own compute function, there is the general
_gtk_css_value_compute() function that then calls a vfunc on the
subtype.
