make structure inheritance explicit
rename VdpGenericHandle to VdpGenericData

it's a data associated with handle, not handle itself. Therefore it should
have 'Data' suffix.
add .device field to all Vdp structs to store VdpDevice handle
protect refcounts of device and pq target with mutexes
stop acquiring presentation queue handle in presentation_thread

since thread will be stopped only on presentation queue destroy, it's
possible use structure directly. There still need in mutex for concurrent
access to queue and conditional variable.
use barrier to ensure worker thread starts accepting before *Create quits
update ChangeLog