avcodec/v4l2_m2m_dec: dequeue frame if input isn't ready

The V4L2M2M API operates asynchronously, so multiple packets can
be enqueued before getting a batch of frames back. Since it was
only possible to receive a frame by submitting another packet,
there wasn't a way to drain those excess output frames from when
avcodec_receive_frame() returned AVERROR(EAGAIN).

Reviewed-by: Andriy Gelman <andriy.gelman@gmail.com>
Signed-off-by: Cameron Gutman <aicommander@gmail.com>

libavcodec/v4l2_m2m_dec.c

@@ -142,8 +142,12 @@ static int v4l2_receive_frame(AVCodecContext *avctx, AVFrame *frame)
 
     if (!s->buf_pkt.size) {
         ret = ff_decode_get_packet(avctx, &s->buf_pkt);
-        if (ret < 0 && ret != AVERROR_EOF)
-            return ret;
+        if (ret < 0) {
+            if (ret == AVERROR(EAGAIN))
+                return ff_v4l2_context_dequeue_frame(capture, frame, 0);
+            else if (ret != AVERROR_EOF)
+                return ret;
+        }
     }
 
     if (s->draining)