tmc: Track offset between tmc driver and mcu position

Track the offset between driver phase and mcu position.  This offset
should be constant as long as neither the driver nor the mcu is reset.
If the offset ever changes, log a warning.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>

klippy/stepper.py

@@ -163,6 +163,7 @@ class MCU_stepper:
         if ret:
             raise error("Internal error in stepcompress")
         self._set_mcu_position(last_pos)
+        self._mcu.get_printer().send_event("stepper:sync_mcu_position", self)
     def set_trapq(self, tq):
         ffi_main, ffi_lib = chelper.get_ffi()
         if tq is None: