- Makefile (OBJS): added ppm.o and ppmimg.o
- ppm.h (load_ppm), ppm.c: PPM file loader, adapted from
eda-tools/schhist/ppmdiff/ppmdiff.c
- ubb-vga.h (img_name, ppmimg), ppmimg.c: PPM image to frame buffer
converter
- ubb-vga2.c (usage, main): the threshold is now set with the option -l
- ubb-vga2.c (usage, main): if a second argument is given, treat it as
a PPM file
- ubb-vga2.c (usage): also documented option -t
