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.
[originally from svn r5979]
