Fix arc acceleration constraint

This fixes an accel constraint violation in Z if you set all maxvel to 8.3333,
all maxaccel to 30, and run this arc, to see the violation at its start
(between the G1 and G2):

G20 G40 G80 G90 G94 G64 F999
G0 X0.2320 Z-0.0771 Y0
G1 X0.2560 Z-0.1203
G18 G2 X0.2620 Y0 Z-0.1134 I-0.0025 K0.0082
M2

I think the key is that this arc is straightish (line-like) and diagonalish
(the endpoints aren't pretty much along just one axis direction).