Limit total stepping speed to avoid overloading the MCU.
This allows us set the the max speeds of the individual axes without considering of the hardware/software
speed.
Because the limit is done over the sum of the stepping speeds of all axes, individual axes can now move much faster.
In particular, this speeds up travel moves, retractions and layer changes.
Homing speeds are not affected by this, they still need to be set sufficiently low.