BitDepthDowngrader plugin initial commit for 1.4.18 nightly

Converts 16 bit data image to 8-bit. Expects true 16-bit images (not 12
& 14 bit). Use only for cameras that do not support native 8-bit
acquisition. Beware: Loss of dynamic range. Rudimentary in terms of
options. For MMgr 1.4.18 nightly and above.