I've dithered a bit in the past about whether or not it's allowable

I've dithered a bit in the past about whether or not it's allowable
to call game_set_size() twice on the same drawstate. Finally, a
definite decision: it isn't. Accordingly, midend.c arranges never to
do so, the devel docs state that puzzles may enforce by assertion
that it never happens, and the four puzzles which care (i.e. use
blitters) do so.

git-svn-id: svn://svn.tartarus.org/sgt/puzzles@6274 cda61777-01e9-0310-a592-d414129be87e