Really implement wxGraphicsContext::GetDPI()

Return the DPI of the associated window, if any, instead of always
returning hard-coded 72*72.

src/common/graphcmn.cpp

@@ -31,6 +31,8 @@
     #include "wx/log.h"
 #endif
 
+#include "wx/display.h"
+
 #ifdef __WXMSW__
     #include "wx/msw/enhmeta.h"
 #endif
@@ -620,8 +622,19 @@ wxDouble wxGraphicsContext::GetAlpha() const
 
 void wxGraphicsContext::GetDPI( wxDouble* dpiX, wxDouble* dpiY)
 {
-    *dpiX = 72.0;
-    *dpiY = 72.0;
+    if ( m_window )
+    {
+        const wxSize ppi = wxDisplay(m_window).GetPPI();
+        *dpiX = ppi.x;
+        *dpiY = ppi.y;
+    }
+    else
+    {
+        // Use some standard DPI value, it doesn't make much sense for the
+        // contexts not using any pixels anyhow.
+        *dpiX = 72.0;
+        *dpiY = 72.0;
+    }
 }
 
 // sets the pen