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?
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