Improve X11 display controls

[MarkRivers]

Implement delay per e-mail from Mark Tate

GrabDisplayDelay(delay)

sets the time between displays where delay (float) is in seconds. The default is 0.1 seconds

in linear display mode
padcom mildisp bitshift on/off offset

Display is 16 bit LUT over a 32 bit space (display wraps around on high bits)
bitshift moves 16 bit LUT left or right
bitshift = 0 displays modulo(0-64k)
bitshift = -4 displays modulo(0-4k)
bitshift = 2 displays modulao(0-256k)

on/off=0 - don't display images
on/off=1 - display images

offset - constant subtracted from image before display


in log mode
padcom loglin loglinflag logscale offset
loglinflag = 0 - use linear display (with last mildisp parameters)
loglinflag = 1 - use log display

logscale - sets max value in greyscale 
e.g. logscale = 6 - max = 10^6 ADU

offset - subtracted before log_10 is computed - haven't used this too much in practice - but could be used if there were a uniform background, say.