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.

[originally from svn r6274]