Incorrect DEC slew calculations

[trishmapow]

Repost here from Discord.

Steps to reproduce (not 100% guaranteed):

• (maybe?) DEC inverted in config
• start at home position in southern hemisphere
• slew north to 0 DEC (or higher?)
• slew anywhere in RA

What happens:
State before second slew: Tracking,--T---,0,28278,1470,154009,+000000,
OATControl sends :XGC12.000*0.000# and receives 17267|-28278
DEC crashes into RA ring (also occurs over ASCOM)

What should happen:
DEC should not move at all, -28278 should be positive.

I haven't dug into the code enough to understand stepper directions, how coordinates are encoded and so on. I ended up rewiring to avoid DEC invert, and the problem did not occur (limited testing). Will update when I have more time.