Add support for a Nokia 5110 LCD (PCD8544)
Make the planner check that the requested PPI is achievable with the current feedrate.
If not, lower the feedrate.
Fix bug that prevented the first pulse from being executed in PPI mode.
Fix bug that prevented the offset from being applied if the steppers were still moving.
Add support for a Z Motor (as opposed to stepper)
Update RX buffer to accomodate smoother (faster) spiral / circle drawing.
Configure the interrupt priorities to give the best chance of performance.
Make the planner FIFO level checking consistent.
Correct PPI slowdown calculation, but don't enforce it.