GtThreshold: bug fix in computing segmentation threshold.

The computed threshold was not a continuous function of the parameters -> very bad!
Imagine if meanvol = 1, and using default values for percentile 1 and 2:
  if stdvol = 1.00001  -> threshold_val = 0.25
  if stdvol = 0.99999  -> threshold_val = 0.8
This is corrected now by using percentile2 only.

Signed-off-by: preischig <preischig@xnovotech.com>