Fix computation of map buffer indexes

Fix computation of map buffer indexes

The correct method to compute indexes into a map buffer for a struct
range is deltx(), delty().

path() used deltax(), deltay() instead, which yield correct results
only for indexes up to half the world size.  Pathes spanning larger
areas were screwed up.

sona(), radmap2(), satmap() also used deltax(), deltay(), but only
with arguments where those yield correct results.

draw_map() used xnorm(), ynorm() instead, which is correct, but less
clear and less efficient.