Rewrote timer code.

The new code is based on timestamps, rather than delays. This eliminates the
need to update every timer in timer_process(), which cost a non-trivial amount
of CPU time on low-end machines.

This involved changes to every user of the timer code, so this change almost
certainly introduces bugs.

src/lpt_dac.c

@@ -30,8 +30,6 @@ static void dac_write_data(uint8_t val, void *p)
 {
         lpt_dac_t *lpt_dac = (lpt_dac_t *)p;
         
-        timer_clock();
-
         if (lpt_dac->is_stereo)
         {
                 if (lpt_dac->channel)