hacked thresholding

hacked thresholding
This is a quick hack using the older recompute the paths using
a soft threshold code. Once we get the opengl line depth path working
most of this can be reverted.

One of key weaknesses is this calls draw_line repeatedly instead of using
the display list.

Also the reason we have the hack is that properly computing the score for
a path is going to take some more refactoring, in this case probably
coming up with some class to handle generating paths from the lists of
FLPs.