Use DPI Aware wxGetSystemMetrics

If no wxWindow is known, use wxTheApp->GetTopWindow().
Also use a wxWindow for all wxSystemSettings::GetMetric calls.

src/generic/odcombo.cpp

@@ -830,7 +830,7 @@ wxSize wxVListBoxComboPopup::GetAdjustedSize( int minWidth, int prefHeight, int
     CalcWidths();
 
     // Take scrollbar into account in width calculations
-    int widestWidth = m_widestWidth + wxSystemSettings::GetMetric(wxSYS_VSCROLL_X);
+    int widestWidth = m_widestWidth + wxSystemSettings::GetMetric(wxSYS_VSCROLL_X, this);
     return wxSize(minWidth > widestWidth ? minWidth : widestWidth,
                   height+2);
 }