A test harness for the convolution code in the probe document.  The
runsh script compiles and runs a 3d convolution test producing a
resulting PNG file, which should match the file correct-3d.png.  More
specifically, the command

	run.sh prog

will compile the file prog.c to an executable prog, which is run to
produce an image file prog.png.  For example,

	run.sh c3d

produces the file c3d.png from the code in c3d.c.