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.

[originally from svn r9021]
[r8997 == 26c669a6b2892b97b4a9bc6842f633291951fdbd]