Alternate braking FETs in COMP_PWM

[jkflying]

Right now only the top FETs do braking duty during COMP_PWM. It would be better if this was alternated between top and bottom FETs to spread the load, and it would also charge the bootstrap capacitors better for NFET based systems.

[sim-]

Right. I was thinking about this, though the high side and low side FETs do have different properties, so alternating might cause output irregularities. Simply swapping things (so that the low side has two FETs on at a time) can be done today by setting HIGH_SIDE_PWM and flipping the high and low side FETs in the .inc file.

I am looking at moving things around to permit braking while still tracking the motor timing, and doing this at around the same time would probably be good.