Fix full closure of avformat on explicit close.

Fix full closure of avformat on explicit close.

JBM wrote:
Creating a video4linux producer like (simplified code):

producer = new Mlt::Producer(*profile, "video4linux2:/dev/video0");
consumer->connect(*producer);
consumer->start();

It works fine. But when I want to stop the capture, there is no way to
stop the video4linux producer. Deleting the producer and the consumer
still leaves the video4linux device open.

After a few hours of struggling, I figured out that some stuff was kept
in the cache. Adding :

mlt_service_cache_purge( MLT_PRODUCER_SERVICE(parent) );

to producer_close (as already done for the qimage producer for example)
fixes the issue.