Music Manager: add suspension functionality

Suspending a tune makes it inaudible, but doesn't change
its timing. During suspension, other tunes may come into
priority. After un-suspending, the tune plays from the
point where it would be if it had not been suspended at
all.

src/music.cpp

@@ -261,6 +261,28 @@ void Music_UnPause(const char* id)
 	}
 }
 
+void Music_Suspend(const char* id)
+{
+	Tune* tune = g_tunes.find(id);
+
+	if (tune)
+	{
+		tune->suspend = true;
+		g_tunes.tick();
+	}
+}
+
+void Music_UnSuspend(const char* id)
+{
+	Tune* tune = g_tunes.find(id);
+
+	if (tune)
+	{
+		tune->suspend = false;
+		g_tunes.tick();
+	}
+}
+
 void Music_PauseAll(void)
 {
 	g_tunes.for_each([](Tune& tune) { tune.pause(); });
@@ -301,6 +323,12 @@ boolean Music_Paused(const char* id)
 	return tune && tune->paused();
 }
 
+boolean Music_Suspended(const char* id)
+{
+	const Tune* tune = g_tunes.find(id);
+	return tune && tune->suspend;
+}
+
 tic_t Music_Elapsed(const char* id)
 {
 	const Tune* tune = g_tunes.find(id);

src/music.h

@@ -75,6 +75,13 @@ void Music_UnPause(const char *id);
 void Music_PauseAll(void);
 void Music_UnPauseAll(void);
 
+// Suspend a tune. The manager will switch to a tune that is
+// not suspended. Upon unsuspending, the tune resumes from
+// the position it would have reached normally (so the
+// duration is not extended like with pausing).
+void Music_Suspend(const char *id);
+void Music_UnSuspend(const char *id);
+
 
 //
 // Change properties. May be called before calling Music_Play.
@@ -117,6 +124,9 @@ boolean Music_Playing(const char *id);
 // Returns true if the tune is paused.
 boolean Music_Paused(const char *id);
 
+// Returns true if the tune is suspended.
+boolean Music_Suspended(const char *id);
+
 // Returns the number of tics elapsed since the start of the
 // tune.
 tic_t Music_Elapsed(const char *id);

src/music_manager.cpp

@@ -49,7 +49,7 @@ void TuneManager::tick()
 	Tune* tune = current_tune();
 
 	std::string old_song = current_song_;
-	current_song_ = tune && tune->playing() ? tune->song : std::string{};
+	current_song_ = tune && tune->playing() && !tune->suspend ? tune->song : std::string{};
 
 	bool changed = current_song_ != old_song;
 

src/music_tune.hpp

@@ -75,6 +75,7 @@ public:
 	bool needs_seek = false;
 	bool resume = false;
 	bool ending = false;
+	bool suspend = false;
 
 	tic_t elapsed() const { return std::max(pause_.value_or(detail::tic_time()), begin_) - begin_; }
 	tic_t time_remaining() const { return end_ - std::min(pause_.value_or(detail::tic_time()), end_); }
@@ -108,14 +109,14 @@ public:
 	{
 		// If this song is not playing, it has lowest
 		// priority.
-		if (!playing())
+		if (!playing() || suspend)
 		{
 			return true;
 		}
 
 		// If the other song is not playing, we automatically
 		// have higher priority.
-		if (!b.playing())
+		if (!b.playing() || b.suspend)
 		{
 			return false;
 		}
@@ -142,6 +143,7 @@ public:
 		needs_seek = true;
 		resume = false;
 		ending = false;
+		suspend = false;
 
 		begin_ = detail::tic_time();
 		end_ = INFTICS;
@@ -162,6 +164,8 @@ public:
 			can_fade_out = true;
 			ending = false;
 		}
+
+		suspend = false;
 	}
 
 	void stop()