libdraw: use pid as initial screenid so one can have more than 25 windows.

sys/src/libdraw/window.c

@@ -22,6 +22,8 @@ allocscreen(Image *image, Image *fill, int public)
 	s = malloc(sizeof(Screen));
 	if(s == 0)
 		return 0;
+	if(!screenid)
+		screenid = getpid();
 	for(try=0; try<25; try++){
 		/* loop until find a free id */
 		a = bufimage(d, 1+4+4+4+1);