qtdemux: support seeking of CENC encrypted streams

qtdemux: support seeking of CENC encrypted streams

When playing a stream that has been protected by DASH CENC, playback
will fail if a seek is performed. Qtdemux produces the error "stream
is protected using cenc, but no cenc protection system information
has been found" and playback stops.

The problem is that gst_qtdemux_reset() gets called as part of the
FLUSH during a seek. This function frees the protection_system_ids
array. When gst_qtdemux_configure_protected_caps() is called after the
seek has completed, the protection_system_ids array is empty and
qtdemux is unable to create the correct output caps for the protected
stream.

This commit changes it to only free the protection_system_ids on
hard resets.

https://bugzilla.gnome.org/show_bug.cgi?id=761787