Previously, for compatibility with GTK 3.0, we allowed specifying
numbers without units and interpreted them as pixels, even when the CSS
specification didn't.
Remove that now that we can break API.
This requires adding code to do math on number values:
gtk_css_number_value_multiply()
and
gtk_css_number_value_try_add()
were added to achieve that.
Some tests are included.
GtkCssNumberValue is now a base class for numbers.
Actual numbers are now implemented in GtkCssDimensionValue. The name is
borrowed from the CSS spec, so there.
This is in preparation for calc(), as calc(50% - 5px) is valid CSS
but has 2 units. Instead, add a function to query a value's dimension
(so we can differentiate lengths from numbers) and add a function to
query if the value contains percentages.
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.
