decoder: fix memory leak when processing interlaced pictures.

decoder: fix memory leak when processing interlaced pictures.

Fix memory leak when processing interlaced pictures and that occurs
because the first field, represented as a GstVideoCodecFrame, never
gets released. i.e. when the picture is completed, this is generally
the case when the second field is successfully decoded, we need to
propagate the GstVideoCodecFrame of the first field to the original
GstVideoDecoder so that it could reclaim memory.

Otherwise, we keep accumulating the first fields into GstVideoDecoder
private frames list until the end-of-stream is reached. The frames
are eventually released there, but too late, i.e. too much memory
may have been consumed.

https://bugzilla.gnome.org/show_bug.cgi?id=701257