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.
Hmm.. this looks weird.. 
Probably it's better to add a virtual function `getoutputdev` to BackgroundRenderer.
What do you say?