We have to return the buffers back to the pool in when stopping to
not mess with the GstBufferPool accounting.
The OMX buffers will be freed when those won't be in charge of the
pool in the chained up call to 'stop'.
Fixes segfaults on finalize and pool not being properly deactivated.
https://bugzilla.gnome.org/show_bug.cgi?id=726337
When using GstOMXBufferPool on an output port, it internally uses
a GPtrArray to manage the GstBuffers instead of the default queue
from the GstBufferPool base class.
In this case GstBufferPool::default_free_buffer is not called when
the pool is stopped. Because the queue is empty. So explicitely
call gst_omx_buffer_pool_free_buffer on each buffer contained in
the GPtrArray.
https://bugzilla.gnome.org/show_bug.cgi?id=726337
