Making DisplayScanner abstract

> We have a DisplayScanner directly using BitBlt like in good old MVC days.
> And we have CanvasCharacterScanner (should be CanvasDisplayScanner) using the alternative Canvas.
> As I understand it, a Canvas enable rendering on other media and with other primitives than BitBlt (opengl or PostScript or...).
>
> CanvasCharacterScanner is mostly unused - relegated in Morphic-Extra, used by an unsent paragraph2:bounds:color: and by hardly ever used RemoteCanvas (Nebraska).
> But I have the feeling it could be interesting.
>
> If that's the case, then we should better make DisplayScanner an abstract class (for various backend) so as to share many methods (rather than copying), and BitBltDisplayScanner would be a subclass with fast direct access to BitBlt, while CanvasDisplayScanner would be a generic class for Canvas case.
>
> Thoughts?