GTK 3 port: don't turn off drawing area double buffering.

GTK 3 port: don't turn off drawing area double buffering.

I think I did this in GTK2 on the basis that our server-side cache
pixmap was double-buffering enough for us - any puzzle which erased a
big area with a background rectangle and then redrew over the top of
it would do so only on the off-screen pixmap, and the updates would
only be copied to the visible window after it was all done.

In GTK3, I don't think there's any need - this is all the usual way
things are done anyway, as far as I can see. So I've turned this call
back off, at least until I hear of a reason why I need it again.