Implement Flush() method for Cairo graphics context

wxCairoContext implementation for wxMSW and wxQT use internally helper surface so there is a need to flush it on demand.

docs/changes.txt

@@ -102,6 +102,7 @@ All (GUI):
 - Fix wxGCDC::Clear() for rotated graphics context.
 - Fix wxGCDC::GetClippingBox() for transformed wxDC (MSW, GTK+).
 - Add support for affine transformation matrix in wxGCDC.
+- Add wxGraphicsContext::Flush() for Cairo renderer.
 
 wxGTK:
 

src/generic/graphicc.cpp

@@ -494,6 +494,7 @@ public:
     virtual void DrawIcon( const wxIcon &icon, wxDouble x, wxDouble y, wxDouble w, wxDouble h ) wxOVERRIDE;
     virtual void PushState() wxOVERRIDE;
     virtual void PopState() wxOVERRIDE;
+    virtual void Flush() wxOVERRIDE;
 
     virtual void GetTextExtent( const wxString &str, wxDouble *width, wxDouble *height,
                                 wxDouble *descent, wxDouble *externalLeading ) const wxOVERRIDE;
@@ -2476,6 +2477,22 @@ void wxCairoContext::PopState()
     cairo_restore(m_context);
 }
 
+void wxCairoContext::Flush()
+{
+#ifdef __WXMSW__
+    if ( m_mswSurface )
+    {
+        cairo_surface_flush(m_mswSurface);
+    }
+#endif
+#ifdef __WXQT__
+    if ( m_qtSurface )
+    {
+        cairo_surface_flush(m_qtSurface);
+    }
+#endif
+}
+
 void wxCairoContext::DrawBitmap( const wxBitmap &bmp, wxDouble x, wxDouble y, wxDouble w, wxDouble h )
 {
     wxGraphicsBitmap bitmap = GetRenderer()->CreateBitmap(bmp);