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_ad1848.h

@@ -19,7 +19,8 @@ typedef struct ad1848_t
         
         int freq;
         
-        int timer_count, timer_latch;
+        pc_timer_t timer;
+	uint64_t timer_latch;
 
         int16_t buffer[MAXSOUNDBUFLEN * 2];
         int pos;