Cunning way to ensure unique solutions in generated Rectangles

Cunning way to ensure unique solutions in generated Rectangles
puzzles. I generate the grid of rectangles as normal, but before I
place the numbers I run it through a non-deterministic solver
algorithm which tries to do as much as it can with as little
information about where the numbers are going to be. The solver
itself narrows down the number placement when it runs out of steam,
but does so as little as possible. Once it reaches a state where it
has ensured solubility, and then the generation algorithm chooses
random number placement from whatever's left.

Occasionally it paints itself into a corner and can't ensure a
unique solution no matter what happens; in that situation we just
have to give up, generate a fresh grid, and try again.

git-svn-id: svn://svn.tartarus.org/sgt/puzzles@5809 cda61777-01e9-0310-a592-d414129be87e