AttachAudioMixedProcessor Utils for easy callback (#215)

Raylib-cs/types/AudioMixed.cs

@@ -0,0 +1,23 @@
+using System;
+using System.Runtime.CompilerServices;
+using System.Runtime.InteropServices;
+
+namespace Raylib_cs;
+
+internal static unsafe class AudioMixed
+{
+    public static AudioCallback<float> Callback = null;
+
+    [UnmanagedCallersOnly(CallConvs = new[]
+    {
+        typeof(CallConvCdecl),
+    })]
+    public static void Processor(void* buffer, uint frames)
+    {
+        // The buffer is stereo audio, so we need to double our frame count.
+        frames = Math.Min(frames * 2, int.MaxValue);
+
+        Span<float> floats = new(buffer, (int)frames);
+        Callback?.Invoke(floats);
+    }
+}