Factor fade amount calculation to separate function

This makes it so to reuse this code, we don't have to copy-paste it.

Additionally, I added a check for the milliseconds being 0, to avoid a
division by zero. Logically and mathematically, if the fade amount is 0
milliseconds, then that means the fade should happen instantly -
however, dividing by zero is undefined (both in math and in C/C++), so
this check needs to be added.

desktop_version/src/Music.cpp

@@ -5,6 +5,7 @@
 #include <stdio.h>
 
 #include "BinaryBlob.h"
+#include "Game.h"
 #include "Map.h"
 #include "UtilityClass.h"
 
@@ -273,10 +274,20 @@ void musicclass::silencedasmusik(void)
 	musicVolume = 0;
 }
 
+void musicclass::setfadeamount(const int fade_ms)
+{
+	if (fade_ms == 0)
+	{
+		FadeVolAmountPerFrame = MIX_MAX_VOLUME;
+		return;
+	}
+	FadeVolAmountPerFrame = MIX_MAX_VOLUME / (fade_ms / game.get_timestep());
+}
+
 void musicclass::fadeMusicVolumeIn(int ms)
 {
 	m_doFadeInVol = true;
-	FadeVolAmountPerFrame =  MIX_MAX_VOLUME / (ms / 33);
+	setfadeamount(ms);
 }
 
 void musicclass::fadeout(const bool quick_fade_ /*= true*/)

desktop_version/src/Music.h

@@ -19,6 +19,7 @@ public:
 	void resume(const int fadein_ms = 0);
 	void haltdasmusik(void);
 	void silencedasmusik(void);
+	void setfadeamount(const int fade_ms);
 	void fadeMusicVolumeIn(int ms);
 	void fadeout(const bool quick_fade_ = true);
 	void fadein(void);