Implement audio forwarding

Use Android Open Accessory 2 to redirect the device audio output to the
computer, creating a new audio input source.

Record this new source and play it to the default output.

<https://source.android.com/devices/accessories/aoa2#audio-support>

app/meson.build

@@ -18,6 +18,13 @@ src = [
     'src/tinyxpm.c',
 ]
 
+if get_option('audio_support')
+    src += [
+        'src/aoa.c',
+        'src/audio.c'
+    ]
+endif
+
 dependencies = [
     dependency('libavformat'),
     dependency('libavcodec'),
@@ -25,6 +32,10 @@ dependencies = [
     dependency('sdl2'),
 ]
 
+if get_option('audio_support')
+    dependencies += dependency('libusb-1.0')
+endif
+
 cc = meson.get_compiler('c')
 
 if host_machine.system() == 'windows'
@@ -83,6 +94,9 @@ conf.set('SKIP_FRAMES', get_option('skip_frames'))
 # enable High DPI support
 conf.set('HIDPI_SUPPORT', get_option('hidpi_support'))
 
+# enable audio support (enable audio forwarding with --forward-audio)
+conf.set('AUDIO_SUPPORT', get_option('audio_support'))
+
 configure_file(configuration: conf, output: 'config.h')
 
 src_dir = include_directories('src')