implemented wxDisplaySizeMM for gtk, msw & motif.

used wxDisplaySize and wxDisplaySizeMM to calculate real screen PPI in
wxPostScriptPrinter.
documented wxDisplaySize and wxDisplaySizeMM.


git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@8414 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/gtk/utilsgtk.cpp

@@ -88,6 +88,12 @@ void wxDisplaySize( int *width, int *height )
     if (height) *height = gdk_screen_height();
 }
 
+void wxDisplaySizeMM( int *width, int *height )
+{
+    if (width) *width = gdk_screen_width_mm();
+    if (height) *height = gdk_screen_height_mm();
+}
+
 void wxGetMousePosition( int* x, int* y )
 {
     gdk_window_get_pointer( (GdkWindow*) NULL, x, y, (GdkModifierType*) NULL );