Refactored the game_size() interface, which was getting really

Refactored the game_size() interface, which was getting really
unpleasant and requiring lots of special cases to be taken care of
by every single game. The new interface exposes an integer `tile
size' or `scale' parameter to the midend and provides two much
simpler routines: one which computes the pixel window size given a
game_params and a tile size, and one which is given a tile size and
must set up a drawstate appropriately. All the rest of the
complexity is handled in the midend, mostly by binary search, so
grubby special cases only have to be dealt with once.

[originally from svn r6059]