Redraw glitch: tiles marked black (at game-over time) were not

Redraw glitch: tiles marked black (at game-over time) were not
redrawn as non-black on undo. Introduce a new flag TILE_IMPOSSIBLE,
so that information about those black markers is cached in the
drawstate and we know when we have to erase them.

In the process I've removed the game_state argument completely from
the subfunction tile_redraw(), which gives me some confidence that
it isn't getting any _more_ privileged information out of it.

git-svn-id: svn://svn.tartarus.org/sgt/puzzles@5979 cda61777-01e9-0310-a592-d414129be87e