Update lessc.inc.php

Fix: color: contrast(#000); generate error because light and dark was not optional, the contrast function not act like lesscss.org documentation.
also the thresold is fixed and not implemented like less.js

this change:
- make optional light color, dark color and threshold
- threshold act like less.js implementation
- add lib_luma (for threshold calculation)
Update lessc.inc.php
rebuild tests