window: Validate style context before first allocation

When we don't do that, we completely botch sizing popups. Not good.

Fixes remaining failing reftests

gtk/gtkwindow.c

@@ -47,6 +47,7 @@
 #include "gtkwidgetprivate.h"
 #include "gtkcontainerprivate.h"
 #include "gtkintl.h"
+#include "gtkstylecontextprivate.h"
 #include "gtktypebuiltins.h"
 #include "a11y/gtkwindowaccessible.h"
 
@@ -4807,6 +4808,10 @@ gtk_window_show (GtkWidget *widget)
 
   need_resize = _gtk_widget_get_alloc_needed (widget) || !gtk_widget_get_realized (widget);
 
+  _gtk_style_context_validate (gtk_widget_get_style_context (widget),
+                               g_get_monotonic_time (),
+                               0);
+
   if (need_resize)
     {
       GtkWindowGeometryInfo *info = gtk_window_get_geometry_info (window, TRUE);