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/vid_ega.h

@@ -39,8 +39,8 @@ typedef struct ega_t
         
         int vres;
         
-        int dispontime, dispofftime;
-        int vidtime;
+        uint64_t dispontime, dispofftime;
+	pc_timer_t timer;
         
         uint8_t scrblank;