Wet & Sticky

The Chalk Wet & Sticky module is derived from the seminal dissertation
"Wet & Stick: A Novel Model for Computer-Based Painting" by Malcom Tunde
Cockshott, and the implementation of that model by Tunde Cockshott,
David England and Kevin Waite. The complete source code to the first
implementation is included in the module_ws/ws and is released under
the terms of the GPL. 

The W&S model is implemented in the following components:

* A color strategy
* A paint op
* A filter

The color strategy implements the canvas; the paint op implements the
application of paint and the filter implements the paint simulation
engine.

This system adds the following interesting capabilities to Chalk:

* Extending the tool options dialog with a widget describing the 
  paint op.

* Extending the paint op class with properties beyond opacity and
  color to a more generic structure with can contain the many different
  properties needed by more complex color models to calculate bitBlt's.

  All the ordinary paint ops still work, but they act as if they are
  applying dry, thin paint, conforming to Cockshott's analysis of the
  Shoup model (which Chalk implemented in the first instance) as a subset
  of the W&S model.

* Adding continuously running filters (either in separate threads or 
  called by a timer) to a particular paint device.

* Adding a new way to mix colour; the older colour selection widgets
  still work, but only give completely dry, infinitely thin paint.

* Creating a layer with a fill of 'substrate' cells -- i.e, filling not just
  with colour, but also with certain calculated amounts of height,
  gravity and absorbency.
