Vector plot: Correctly compute thresholding scaling relative to colour data value
Vector plot: Correctly compute thresholding scaling relative to colour data value

* This threshold scaling relative to colour value is displayed within the colourbar
* Clamp the range to be the intersection of the windowing

MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3