* No longer keep a global variable with the current stage number, but pass it

 * No longer keep a global variable with the current stage number, but pass it
   as an argument to the functions that need it. This also replaces the
   stage_odd parameter.
 * No longer loop through the needed stages, since the Montium sequencer can't
   change the twiddle memory mask dynamically. Instead, call the (new)
   do_regular_stage() function four times manually, with constant stage
   numbers that the optimizer can roll out.