G-codes in case of single extruder multiple material setup:
At the start of the print, the filament_gcode_start is executed
for the active extruder only, and the filament_gcode_start /
filament_gcode_end are then executed at each tool change.
When the Prusa MM wipe tower is active, the tool changes are handled
a bit differently: M900 K0 is emited before the wipe tower extrusions start,
and the filament_gcode_start code is executed after the wipe tower extrusions
are done. This rule effectively disables the linear advance over the wipe tower.
Implements https://github.com/prusa3d/Slic3r/issues/568
3996535e5d