General robustness patch from James Harvey:

 - most game_size() functions now work in doubles internally and
   round to nearest, meaning that they have less tendency to try to
   alter a size they returned happily from a previous call
 - couple of fiddly fixes (memory leaks, precautionary casts in
   printf argument lists)
 - midend_deserialise() now constructs an appropriate drawstate,
   which I can't think how I overlooked myself since I _thought_ I
   went through the entire midend structure field by field!

git-svn-id: svn://svn.tartarus.org/sgt/puzzles@6041 cda61777-01e9-0310-a592-d414129be87e