When values are computed, they might depend on various other values and
we need to track this so we can update the values when those other
values change. This is the first step in making that happen.
This patch does not do any dependency tracking at all, instead it uses
GTK_CSS_DEPENDS_ON_EVERYTHING as a sort of FIXME.
This is a reorganization of how value computing should be done.
Previously the GtkCssStyleProperty.compute vfunc was supposed to take
care of special cases when it needed those for computation. However,
this proved to be very complicated in cases where values were nested and
only the last value (of a common type) needed to be special cased.
A common example for this was the fallback handling for unresolvable
colors.
Now, we pass the property's ID along with all compute functions so we
can do the special casing where it's necessary.
Note that no actual changes happen in this commit. This will happen in
follow-ups.
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.
Returns a value that transitions between start and end or %NULL if the
values cannot be transitioned.
So far, all implementations but numbers and rgba return NULL.
