Fix concurrent modification error modifying sound volume

This is most likely caused by another mod inadvertently modifying
the `instanceToChannel` map (by playing a sound?) from another thread.

platforms/common/src/main/java/dynamic_fps/impl/mixin/SoundEngineMixin.java

@@ -1,14 +1,12 @@
 package dynamic_fps.impl.mixin;
 
 import java.util.Map;
+import java.util.concurrent.ConcurrentHashMap;
 
 import dynamic_fps.impl.feature.volume.SmoothVolumeHandler;
 import net.minecraft.client.Minecraft;
 import org.jetbrains.annotations.Nullable;
-import org.spongepowered.asm.mixin.Final;
-import org.spongepowered.asm.mixin.Mixin;
-import org.spongepowered.asm.mixin.Shadow;
-import org.spongepowered.asm.mixin.Unique;
+import org.spongepowered.asm.mixin.*;
 import org.spongepowered.asm.mixin.injection.At;
 import org.spongepowered.asm.mixin.injection.Inject;
 import org.spongepowered.asm.mixin.injection.callback.CallbackInfo;
@@ -39,6 +37,7 @@ public class SoundEngineMixin implements DuckSoundEngine {
 
 	@Shadow
 	@Final
+	@Mutable
 	private Map<SoundInstance, ChannelAccess.ChannelHandle> instanceToChannel;
 
 	@Shadow
@@ -49,6 +48,13 @@ private float calculateVolume(SoundInstance instance) {
 	@Unique
 	private static final Minecraft dynamic_fps$minecraft = Minecraft.getInstance();
 
+	@Inject(method = "<init>", at = @At("TAIL"))
+	private void init(CallbackInfo callbackInfo) {
+		// Fix crash from another unknown mod mutating this
+		// While we're iterating over it in the `dynamic_fps$updateVolume` method
+		this.instanceToChannel = new ConcurrentHashMap<>(this.instanceToChannel);
+	}
+
 	@Override
 	public void dynamic_fps$updateVolume(SoundSource source) {
 		if (!this.loaded) {