More ES1 gutenprint goodness

Gutenprint has accepted my second patch, so it now has a working Selphy ES1 raster driver. Unfortunately, it still requires a custom print spooler, but I'm now one step closer.

es_print_assist.c is now updated to properly poll the printer status, so it can now take the raw dump from Gutenprint and shove it out to the printer with minimal delay.

The third step will be to rework it so that it can deal with an arbitrary file on stdin, properly parsing the dumpfile to determine length and paper type.. and for step four, adapting it into a proper CUPS backend. Yay.