Log Message:
gst-plugins1-libav: fix mp4 playback

From Jana Saout in https://bugzilla.gnome.org/show_bug.cgi?id=789193

Not meant as "fix" but as temporary workaround for people bothered
by this issue:

This quick & dirty hack replaces the deprecated call to
avcodec_decode_video2 by calls to avcodec_send_packet and
avcodec_receive_frame.

Note that this is hack is really ugly, because the API really isn't
meant to be used this way (the assumption is that each call to
avcodec_send_packet will produce at most one new frame) and also
the error handling and probably other cases are not handled correctly.

However, MP4 videos seem to play fine for me.