Draw_VL_ab ($F3D8)
This routine draws vectors between the set of (y,x) points pointed to by the X register. The vector list has the following format:
rel y, rel x, rel y, rel x, ...
ENTRY:
DP = $D0
A-reg = the number of vectors to draw
B-reg = the scale factor
X-reg points to the vector list
EXIT:
X-reg points to next byte after list
D-reg trashed