Fix it so that it checks return status of fprintf() and friends when 
saving files. Probably need a setjmp() to deal with this.

Clean up obj_line and obj_spline - too much code duplication there.

Checkpointing using changes

Scale, Rotate, Mirror

Multiple buffer handling

Enhancements:
	ordering of gels so that we can use filled regions, Raise, Lower
	initially fill regions with setgray
	later fill regions with general bitmaps - load and save bitmaps
	grab bitmaps from the screen
	macros, groups
	
Generalize so that all global variables are in a struct to which a
pointer is passed for all routines - eg. picWin becomes pic->win,
picDpy becomes pic->dpy, gcInvert becomes pic->gcInvert, etc. This is
a step towards widgetification of xpic.
