Until now, Same Game has been the only puzzle in this collection

Until now, Same Game has been the only puzzle in this collection
which is unable to guarantee that every grid it generates can be
solved. So I'm eliminating that exception: this checkin contains a
more sophisticated grid generator which does guarantee solubility.
It's a bit slow (most noticeably on the 15x10c3 preset), and the
quality of the generated grids is slightly weird (a tendency toward
small regions rather than large sweeping areas of contiguous
colour); however, I'm willing to see the latter as a feature for
now, since making the game more challenging while simultaneously
guaranteeing it to be possible sounds like an all-round win to me.

From now on I'm raising my standards for contributions to this
collection. I made this fix to Same Game because I heard a user
_automatically assume_ that any puzzle in my collection would not be
so uncouth as to generate an impossible grid; as of this checkin
that's actually true, and I intend to maintain that standard of
quality henceforth.

(Guaranteeing a _unique_ solution is more of an optional extra,
since there are many games for which it isn't a meaningful concept
or isn't particularly desirable. Which is not to say that _some_
games wouldn't be of unacceptably low quality if they failed to
guarantee uniqueness; it depends on the game.)

git-svn-id: svn://svn.tartarus.org/sgt/puzzles@6124 cda61777-01e9-0310-a592-d414129be87e