avcodec/v4l2_m2m_dec: set pkt_dts on decoded frames to NOPTS

Without this ffmpeg will attempt to copy the dts from the
most recently enqueued packet into the most recently dequeued
frame, which does not account for the buffering inside v4l2
and is not accurate.
Signed-off-by: Aman Gupta <aman@tmm1.net>

libavcodec/v4l2_buffers.c

@@ -420,6 +420,7 @@ int ff_v4l2_buffer_buf_to_avframe(AVFrame *frame, V4L2Buffer *avbuf)
     frame->color_range = v4l2_get_color_range(avbuf);
     frame->color_trc = v4l2_get_color_trc(avbuf);
     frame->pts = v4l2_get_pts(avbuf);
+    frame->pkt_dts = AV_NOPTS_VALUE;
 
     /* these two values are updated also during re-init in v4l2_process_driver_event */
     frame->height = avbuf->context->height;

libavcodec/v4l2_m2m_dec.c

@@ -242,6 +242,7 @@ static const AVOption options[] = {
         .close          = v4l2_decode_close, \
         .bsfs           = bsf_name, \
         .capabilities   = AV_CODEC_CAP_HARDWARE | AV_CODEC_CAP_DELAY | AV_CODEC_CAP_AVOID_PROBING, \
+        .caps_internal  = FF_CODEC_CAP_SETS_PKT_DTS, \
         .wrapper_name   = "v4l2m2m", \
     };