[client] usb audio: emulate UAC2 playback

Add a fixed-format USB Audio Class 2 playback device for the
client-side USB redirection bridge.

Advertise stereo 48 kHz IEEE float audio and handle enumeration,
configuration, alternate settings, clock requests, and isochronous data.

client/src/usb_audio.h

@@ -0,0 +1,50 @@
+/**
+ * Looking Glass
+ * Copyright © 2017-2026 The Looking Glass Authors
+ * https://looking-glass.io
+ *
+ * This program is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License as published by the Free
+ * Software Foundation; either version 2 of the License, or (at your option)
+ * any later version.
+ *
+ * This program is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for
+ * more details.
+ *
+ * You should have received a copy of the GNU General Public License along
+ * with this program; if not, write to the Free Software Foundation, Inc., 59
+ * Temple Place, Suite 330, Boston, MA 02111-1307 USA
+ */
+
+#ifndef _H_LG_CLIENT_USB_AUDIO_
+#define _H_LG_CLIENT_USB_AUDIO_
+
+#include "usbredir.h"
+
+#include <stddef.h>
+
+#define LG_USB_AUDIO_SAMPLE_RATE 48000
+#define LG_USB_AUDIO_CHANNELS 2
+
+typedef struct LG_USBAudio LG_USBAudio;
+
+typedef struct LG_USBAudioEventOps
+{
+  void (*start)(void * opaque);
+  void (*stop)(void * opaque);
+
+  /* Data is borrowed interleaved stereo 32-bit IEEE float, little endian. */
+  void (*data)(void * opaque, const void * data, size_t frames);
+}
+LG_USBAudioEventOps;
+
+LG_USBAudio * lgUsbAudio_create(
+    const LG_USBAudioEventOps * events, void * eventOpaque);
+/* Destroy the LG_USBRedir using this device before destroying the device. */
+void lgUsbAudio_destroy(LG_USBAudio * audio);
+
+const LG_USBRedirDeviceOps * lgUsbAudio_deviceOps(void);
+
+#endif