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/common/gdicmn.cpp

@@ -691,6 +691,13 @@ wxSize wxGetDisplaySize()
     return wxSize(x, y);
 }
 
+wxSize wxGetDisplaySizeMM()
+{
+    int x, y;
+    wxDisplaySizeMM(& x, & y);
+    return wxSize(x, y);
+}
+
 wxResourceCache::~wxResourceCache ()
 {
     wxNode *node = First ();