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/sound_opl.h

@@ -4,8 +4,7 @@ typedef struct opl_t
 {
         int chip_nr[2];
         
-        int timers[2][2];
-        int timers_enable[2][2];
+        pc_timer_t timers[2][2];
 
         int16_t filtbuf[2];