lavc: add a sample_aspect_ratio field to AVFrame

The sample aspect ratio is a per-frame property, so it makes sense to
define it in AVFrame rather than in the codec/stream context.
Simplify application-level sample aspect ratio information extraction,
and allow further simplifications.

libavcodec/pthread.c

@@ -599,6 +599,7 @@ int ff_thread_decode_frame(AVCodecContext *avctx,
         *picture = p->frame;
         *got_picture_ptr = p->got_frame;
         picture->pkt_dts = p->avpkt.dts;
+        picture->sample_aspect_ratio = avctx->sample_aspect_ratio;
 
         /*
          * A later call with avkpt->size == 0 may loop over all threads,