Reuse wxDC::GetMultiLineTextExtent() in wxGridCellStringRenderer

There is no need to reimplement the same logic in wxGrid code when we
already have a perfectly cromulent way to do it in wxDC itself (which
hadn't existed when this code was originally written, explaining why it
wasn't used here before).

This makes the code shorter and also a bit faster, as we avoid using
wxStringTokenizer unnecessarily.

No changes in behaviour.

src/generic/gridctrl.cpp

@@ -557,18 +557,8 @@ wxSize wxGridCellStringRenderer::DoGetBestSize(const wxGridCellAttr& attr,
                                                wxDC& dc,
                                                const wxString& text)
 {
-    wxCoord x = 0, y = 0, max_x = 0;
     dc.SetFont(attr.GetFont());
-    wxStringTokenizer tk(text, wxT('\n'));
-    while ( tk.HasMoreTokens() )
-    {
-        dc.GetTextExtent(tk.GetNextToken(), &x, &y);
-        max_x = wxMax(max_x, x);
-    }
-
-    y *= 1 + text.Freq(wxT('\n')); // multiply by the number of lines.
-
-    return wxSize(max_x, y);
+    return dc.GetMultiLineTextExtent(text);
 }
 
 wxSize wxGridCellStringRenderer::GetBestSize(wxGrid& grid,