atrac1: decode mono audio directly to output buffer

For stereo we need to use intermediate planar buffers, but mono does not need
to be deinterleaved so the output buffer can be used directly.

libavcodec/atrac1.c

@@ -310,15 +310,11 @@ static int atrac1_decode_frame(AVCodecContext *avctx, void *data,
         ret = at1_imdct_block(su, q);
         if (ret < 0)
             return ret;
-        at1_subband_synthesis(q, su, q->out_samples[ch]);
+        at1_subband_synthesis(q, su, q->channels == 1 ? samples : q->out_samples[ch]);
     }
 
     /* interleave; FIXME, should create/use a DSP function */
-    if (q->channels == 1) {
-        /* mono */
-        memcpy(samples, q->out_samples[0], AT1_SU_SAMPLES * 4);
-    } else {
-        /* stereo */
+    if (q->channels == 2) {
         for (i = 0; i < AT1_SU_SAMPLES; i++) {
             samples[i * 2]     = q->out_samples[0][i];
             samples[i * 2 + 1] = q->out_samples[1][i];