Add Gain Filter

Util/Audio/Filter/Gain.cs

@@ -0,0 +1,22 @@
+using System;
+
+namespace ADOFAIRenderer.Util.Audio.Filter
+{
+    public class Gain : IAudioFilter
+    {
+        public float Level = 0f;
+
+        public void Initialize(int sampleRate, int channels) { }
+
+        public float ProcessSample(float sample, int channel) =>
+            sample * (float)Math.Pow(10, Level / 20);
+
+        public void ProcessBlock(Span<float> buffer)
+        {
+            for (var i = 0; i < buffer.Length; i++)
+                buffer[i] = ProcessSample(buffer[i], 0); 
+        }
+
+        public void Reset() { }
+    }
+}