txt2xbm: convert text to XBM, using ghostscript
stretch: resample XBM with rotation correction (x-axis only)
xbm2gp: convert XBM to gnuplot, taking into account rotation
plot: plot xbm2gp output
t: run all the above to display "TEST"
Instead of a counter, which worked okay with 16 LEDs but is useless with
all 64 of them, we now show a filling/emptying bar. The update speed is
high enough that rotation results in about a quarter-circle pattern.
