animations: Start transitions from the current value

... instead of from the intrinsic value. This way, we respect running
animations.

Note that the concept of "reversing" transitions is not implemented yet.

gtk/gtkcsscomputedvalues.c

@@ -411,7 +411,7 @@ gtk_css_computed_values_create_css_transitions (GtkCssComputedValues *values,
       else
         {
           animation = _gtk_css_transition_new (i,
-                                               start,
+                                               _gtk_css_computed_values_get_value (source, i),
                                                _gtk_css_array_value_get_nth (timing_functions, i),
                                                timestamp + delay * G_USEC_PER_SEC,
                                                timestamp + (delay + duration) * G_USEC_PER_SEC);