#36 Implement Mezzotint plug-in

[Maurits Rijk]

Photoshop has a plug-in called Mezzotint (in Pixelate
menu). This plug-in does two things:

1) all rgb values are mapped to either 0 or 255. For
example rgb(13, 129, 44) becomes (0, 255, 0)

2) it adds black and white noise which can have several
types:

Fine/Medium/Grainy/Coarse dots
Short/Medium/Long lines and strokes

This functionality can probably be reproduced by a
combination of existing GIMP plug-ins, but having a
plug-in with exactly this functionality will make it
easier for (ex-)Photoshop users.