Add Glow Over/Under LUT

This LUT is popular in confocal microscopy because under- and
oversaturated pixels stand out in discrete colors, with an otherwise
continuous color table.

Thanks to the IcyLyd on the Icy forums for pointing this out:
https://groups.google.com/d/msg/icy-software/97uGGCmMI5M/TwB_nfJCwlwJ