Fixes to r8997: firstly, move the fix out of #ifdef CAIRO so that it

applies to both drawing mechanisms, and secondly, make sure to paint
the spare pieces of window in the _puzzle's_ background colour
rather than the (sometimes slightly different) system default.

git-svn-id: svn://svn.tartarus.org/sgt/puzzles@9021 cda61777-01e9-0310-a592-d414129be87e