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_cga.h

@@ -18,8 +18,8 @@ typedef struct cga_t
         uint16_t ma, maback;
         int oddeven;
 
-        int dispontime, dispofftime;
-        int vidtime;
+        uint64_t dispontime, dispofftime;
+	pc_timer_t timer;
         
         int firstline, lastline;