avplay: apply the stream sample_aspect_ratio to decoded video frames

If there is a sample_aspect_ratio in the stream, then apply it to every
decoded frame in the same way as avconv does. This also makes sure that
the avfilter chain has access to the aspect ratio.

Signed-off-by: Vladimir Pantelic <vladoman@gmail.com>
Signed-off-by: Anton Khirnov <anton@khirnov.net>