Use wxBufferedPaintDC for wxStyledTextCtrl drawing in wxGTK.

GTK+ doesn't seem to use double buffering for Scintilla window for some
reason, resulting in bad flicker when it's updated, e.g. when the user types
into it.

Force the use of double buffering at wxWidgets level by using
wxBufferedPaintDC for painting it. This may be inefficient if GTK+ does double
buffer it in some cases but at least it gets rid of the flicker.

Closes #12704, #14828.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@73001 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/stc/stc.cpp

@@ -53,6 +53,10 @@
     #include "wx/ffile.h"
 #endif
 
+#ifdef __WXGTK__
+    #include "wx/dcbuffer.h"
+#endif
+
 #include "ScintillaWX.h"
 
 //----------------------------------------------------------------------
@@ -4612,7 +4616,11 @@ void wxStyledTextCtrl::AppendTextRaw(const char* text, int length)
 // Event handlers
 
 void wxStyledTextCtrl::OnPaint(wxPaintEvent& WXUNUSED(evt)) {
+#ifdef __WXGTK__
+    wxBufferedPaintDC dc(this);
+#else
     wxPaintDC dc(this);
+#endif
     m_swx->DoPaint(&dc, GetUpdateRegion().GetBox());
 }