css: Introduce _gtk_css_value_compute()

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.

gtk/gtkcssenginevalue.c

@@ -34,6 +34,13 @@ gtk_css_value_engine_free (GtkCssValue *value)
   g_slice_free (GtkCssValue, value);
 }
 
+static GtkCssValue *
+gtk_css_value_engine_compute (GtkCssValue     *value,
+                              GtkStyleContext *context)
+{
+  return _gtk_css_value_ref (value);
+}
+
 static gboolean
 gtk_css_value_engine_equal (const GtkCssValue *value1,
                             const GtkCssValue *value2)
@@ -67,6 +74,7 @@ gtk_css_value_engine_print (const GtkCssValue *value,
 
 static const GtkCssValueClass GTK_CSS_VALUE_ENGINE = {
   gtk_css_value_engine_free,
+  gtk_css_value_engine_compute,
   gtk_css_value_engine_equal,
   gtk_css_value_engine_transition,
   gtk_css_value_engine_print