appears stable in limited testing.
- Allocate the picture private, avoiding segfault.
- Wrap PaintWindow to draw the background/border to the backing pixmap
    (based on Deron Johnson's comptran.c).
- Set the x_off/y_off returns to translate coordinates properly.
- Don't bother allocating temporary areas for the modified coordinates.
    Layers above are responsible for handling lower layers changing the
    arguments, so cw doesn't have to worry about it. mibstore.c has to do
    the allocation because it calls down twice (front buffer and backing
    store). (Suggested by keithp)
- Handle the mode argument to PolyPoint, Polylines, and FillPolygon.
- Remove some dead elements in the cw privates.
- Kill a prototype warning in compinit.c by adding the cw.h header.