audio filters moved to its own files

src/net/torvald/terrarum/audio/dsp/Gain.kt

@@ -0,0 +1,12 @@
+package net.torvald.terrarum.audio.dsp
+
+import net.torvald.terrarum.audio.TerrarumAudioMixerTrack
+
+class Gain(var gain: Float): TerrarumAudioFilter() {
+    override fun thru(inbuf: List<FloatArray>, outbuf: List<FloatArray>) {
+        for (i in 0 until TerrarumAudioMixerTrack.BUFFER_SIZE / 4) {
+            outbuf[0][i] = inbuf[0][i] * gain
+            outbuf[1][i] = inbuf[1][i] * gain
+        }
+    }
+}