vaapidecode: adopt non-deprecrated glib locking primitive pattern.

vaapidecode: adopt non-deprecrated glib locking primitive pattern.

The use of heap allocated GMutex/GCond is deprecated. Instead place them
inside the structure they are locking.

These changes switch to use g_mutex_init/g_cond_init rather than the heap
allocation functions.

Because we cannot test for a NULL pointer for the GMutex/GCond we must
initialise inside the GObject _init function and clear inside the _finalize
which is guaranteed to only be called once and after the object is no longer
in use.