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opl: Process MIDI 'set tempo' meta events.

The MIDI format includes a special meta event to set the tempo of
playback, and some WADs depend on this - notably the music in Alien
Vendetta. Move the variables controlling tempo to the global scope
(they are not per-track as I previously thought) and set when the
tempo events are encountered.

This is some progress towards resolving #334, but that bug is not
yet completely fixed, because the tempo change does not
retroactively apply to OPL timers that have already been set.

--- a/src/i_oplmusic.c

+++ b/src/i_oplmusic.c

@@ -46,6 +46,9 @@

 #define PERCUSSION_LOG_LEN 16

+// TODO: Figure out why this is needed.

+#define TEMPO_FUDGE_FACTOR 0.26

+

 typedef struct

 {

     byte tremolo;

@@ -103,11 +106,6 @@

     // Track iterator used to read new events.

     midi_track_iter_t *iter;

-

-    // Tempo control variables

-

-    unsigned int ticks_per_beat;

-    unsigned int ms_per_beat;

 } opl_track_data_t;

 typedef struct opl_voice_s opl_voice_t;

@@ -324,6 +322,11 @@

 static unsigned int running_tracks = 0;

 static boolean song_looping;

+// Tempo control variables

+

+static unsigned int ticks_per_beat;

+static unsigned int us_per_beat;

+

 // Mini-log of recently played percussion instruments:

 static uint8_t last_perc[PERCUSSION_LOG_LEN];

@@ -1017,6 +1020,9 @@

 static void MetaEvent(opl_track_data_t *track, midi_event_t *event)

 {

+    byte *data = event->data.meta.data;

+    unsigned int data_len = event->data.meta.length;

+

     switch (event->data.meta.type)

     {

         // Things we can just ignore.

@@ -1032,6 +1038,13 @@

         case MIDI_META_SEQUENCER_SPECIFIC:

             break;

+        case MIDI_META_SET_TEMPO:

+            if (data_len == 3)

+            {

+                us_per_beat = (data[0] << 16) | (data[1] << 8) | data[2];

+            }

+            break;

+

         // End of track - actually handled when we run out of events

         // in the track, see below.

@@ -1161,7 +1174,7 @@

     // Get the number of milliseconds until the next event.

     nticks = MIDI_GetDeltaTime(track->iter);

-    ms = (nticks * track->ms_per_beat) / track->ticks_per_beat;

+    ms = (nticks * us_per_beat * TEMPO_FUDGE_FACTOR) / ticks_per_beat;

     total += ms;

     // Set a timer to be invoked when the next event is

@@ -1190,13 +1203,7 @@

     track = &tracks[track_num];

     track->iter = MIDI_IterateTrack(file, track_num);

-    track->ticks_per_beat = MIDI_GetFileTimeDivision(file);

-    // Default is 120 bpm.

-    // TODO: this is wrong

-

-    track->ms_per_beat = 500 * 260;

-

     for (i=0; i<MIDI_CHANNELS_PER_TRACK; ++i)

     {

         InitChannel(track, &track->channels[i]);

@@ -1228,6 +1235,13 @@

     num_tracks = MIDI_NumTracks(file);

     running_tracks = num_tracks;

     song_looping = looping;

+

+    ticks_per_beat = MIDI_GetFileTimeDivision(file);

+

+    // Default is 120 bpm.

+    // TODO: this is wrong

+

+    us_per_beat = 500 * 1000;

     for (i=0; i<num_tracks; ++i)

     {