flush Cairo surface before converting to wxImage

(cherry picked from commit cd27b73237)

src/generic/graphicc.cpp

@@ -1549,6 +1549,7 @@ wxImage wxCairoBitmapData::ConvertToImage() const
     }
 
     // Prepare for copying data.
+    cairo_surface_flush(m_surface);
     const wxUint32* src = (wxUint32*)cairo_image_surface_get_data(m_surface);
     wxCHECK_MSG( src, wxNullImage, wxS("Failed to get Cairo surface data.") );