ximagesink,xvimagesink: handle drain query

Submitted by Julien Isorce @cap

Link to original bug (#786738)

Description

Created attachment 358306
xvimagesink: unref current image on a drain query

So that an upstream element can claim all buffers to return to its buffer pool.

Patch 358306, "xvimagesink: unref current image on a drain query":
0001-xvimagesink-unref-current-image-on-a-drain-query.patch