fix 16-bit PGM crash bug

fix PNG loading bug for interlaced depth 1 grayscale images with a
background colour, and improve handling of grayscale and paletted PNG
images with transparency

fix crash on interactive zoom with -clip set

fix bug where -fillscreen only applies to first image

make MIT-SHM conditional for systems that don't have it

document -delete, -iscale, -focus, -cache, '>', '<', and '='

and NEVER MIND tweaking send.c for performance.  I mean, really, WHO ON
EARTH CARES?  Except me, I mean.

add vicar reader, copyright Noel Gorelick, gorelick@asu.edu, description:
'VICAR: a general purpose multi-dimensional image processing package
developed by NASA/JPL.'

make fast image scaling a subfeature of -zoom, eliminate -iscale

add a TIFF reader?

add configurable external filters, action programs (making -delete
redundant) and maybe readers
