avcodec/v4l2_buffers: don't prevent enqueue capture buffer to driver

Enqueue/dequeue of the capture buffers should continue while draining.

Reference: linux/Documentation/media/uapi/v4l/dev-decoder.rst

    "The client must continue to handle both queues independently,
     similarly to normal decode operation. This includes:
     ...
     - queuing and dequeuing CAPTURE buffers, until a buffer marked with
       the V4L2_BUF_FLAG_LAST flag is dequeued"
Signed-off-by: Ming Qian <ming.qian@nxp.com>
Signed-off-by: Andriy Gelman <andriy.gelman@gmail.com>

libavcodec/v4l2_buffers.c

@@ -222,7 +222,7 @@ static void v4l2_free_buffer(void *opaque, uint8_t *unused)
             if (!atomic_load(&s->refcount))
                 sem_post(&s->refsync);
         } else {
-            if (s->draining) {
+            if (s->draining && V4L2_TYPE_IS_OUTPUT(avbuf->context->type)) {
                 /* no need to queue more buffers to the driver */
                 avbuf->status = V4L2BUF_AVAILABLE;
             }