Implement proof output

This patch add --proof option to output a proof version. If a positive value is specified, texts are drawn on both text layer and background image  for comparison. If  2 is specified, texts on background are in different colors, usually red for filling, blue for stroking.
Yes it is. I think it's even better to add a `outputdev` parameter to `proof_begin_string()` `proof_end_text_object()`.