Improve DPI handling in wxSTC

Adjusting the zoom level does not work correctly. It could lead to ever
increasing zoom.
Instead, set the correct DPI of the underlying device context.

samples/stc/edit.cpp

@@ -196,7 +196,7 @@ Edit::Edit (wxWindow *parent, wxWindowID id,
 
     // miscellaneous
     m_LineNrMargin = TextWidth (wxSTC_STYLE_LINENUMBER, "_999999");
-    m_FoldingMargin = 16;
+    m_FoldingMargin = FromDIP(16);
     CmdKeyClear (wxSTC_KEY_TAB, 0); // this is done by the menu accelerator key
     SetLayoutCache (wxSTC_CACHE_PAGE);
     UsePopUp(wxSTC_POPUP_ALL);