audio: Fix audio stream callback calculations when future buffer has space.

We were subtracting backwards.  :/

src/audio/SDL_audiocvt.c

@@ -1038,7 +1038,7 @@ int SDL_GetAudioStreamData(SDL_AudioStream *stream, void *voidbuf, int len)
             // calculate difference in dataset size after resampling. Use a Uint64 so the multiplication doesn't overflow.
             approx_request = (int) (size_t) ((((Uint64) approx_request) * stream->src_spec.freq) / stream->dst_spec.freq);
             if (!stream->flushed) {  // do we need to fill the future buffer to accomodate this, too?
-                approx_request += stream->future_buffer_filled_frames - stream->resampler_padding_frames;
+                approx_request += stream->resampler_padding_frames - stream->future_buffer_filled_frames;
             }
         }