1) Writeable icons with a sprite too - caret positioning and
   text updating all wrong.  It's as if the width of the
   sprite is not accounted for, sort of.

2) Wimp_DragBox 6 (variable size box) has a bug; the constraint
   rectangle only works as expected if the initial size of the rubber
   rectangle is zero.  If non-zero, then the constraint is applied as for
   a non-stretchy rectangle of that size.

3) The wimp can paint outside an icon's bounding box!!  I've noticed
   this with text+sprite icons like radio buttons.  It even happens inside
   FormEd so it's not just when using Wimp_PlotIcon.

4) No way to give an explicit window handle to Wimp_PlotIcon
   hence no way to print it!

5) Pushed-in buttons are not safe when the program exits!! The Wimp
   blows up shortly afterwards...
