Added a GetPendingBytes method to the audio backend.

This will (eventually) make SDL_GetQueuedAudioSize() more accurate, and thus
reduce latency. Right now this isn't implemented anywhere, so we assume data
fed to the audio callback is consumed by the hardware and immediately played
to completion.

src/audio/SDL_audio.c

@@ -178,6 +178,12 @@ SDL_AudioPlayDevice_Default(_THIS)
 {                               /* no-op. */
 }
 
+static int
+SDL_AudioGetPendingBytes_Default(_THIS)
+{
+    return 0;
+}
+
 static Uint8 *
 SDL_AudioGetDeviceBuf_Default(_THIS)
 {
@@ -253,6 +259,7 @@ finalize_audio_entry_points(void)
     FILL_STUB(ThreadInit);
     FILL_STUB(WaitDevice);
     FILL_STUB(PlayDevice);
+    FILL_STUB(GetPendingBytes);
     FILL_STUB(GetDeviceBuf);
     FILL_STUB(WaitDone);
     FILL_STUB(CloseDevice);
@@ -471,7 +478,7 @@ SDL_GetQueuedAudioSize(SDL_AudioDeviceID devid)
     SDL_AudioDevice *device = get_audio_device(devid);
     if (device) {
         current_audio.impl.LockDevice(device);
-        retval = device->queued_bytes;
+        retval = device->queued_bytes + current_audio.impl.GetPendingBytes(device);
         current_audio.impl.UnlockDevice(device);
     }
 

src/audio/SDL_sysaudio.h

@@ -60,6 +60,7 @@ typedef struct SDL_AudioDriverImpl
     void (*ThreadInit) (_THIS); /* Called by audio thread at start */
     void (*WaitDevice) (_THIS);
     void (*PlayDevice) (_THIS);
+    int (*GetPendingBytes) (_THIS);
     Uint8 *(*GetDeviceBuf) (_THIS);
     void (*WaitDone) (_THIS);
     void (*CloseDevice) (_THIS);