New puzzle: `Tents'. Requires a potentially shared algorithms module

maxflow.c. Also in this checkin, fixes to the OS X and GTK back ends
to get ALIGN_VNORMAL right. This is the first time I've used it! :-)

[originally from svn r6390]

osx.m

@@ -1341,6 +1341,8 @@ static void osx_draw_text(void *handle, int x, int y, int fonttype,
 	point.x -= size.width / 2;
     if (align & ALIGN_VCENTRE)
 	point.y -= size.height / 2;
+    else
+	point.y -= size.height;
 
     [string drawAtPoint:point withAttributes:attr];
 }