fix decklink producer dropping frames after few hours

fix decklink producer dropping frames after few hours

mlt_cache_put() was not scalable when using position as a key into the
cache. Its reliance upon mlt_properties for reference-counting cache
items caused its hash tabe to grow too large after a few hundred
thousand frames. This version uses the new mlt_cache_put_frame, which is
simpler and more scalable by only using the cache arrays and copying
data.