codec/rfx: fix multithreaded encoder

Some component of the encoder chain (I suspect the rlgr encoder) expects
the output buffer to be zeroed. The multithreaded RemoteFX encoder uses
wStreams from the StreamPool which are reused and not zeroed out of
course. For now, in order to prevent data corruption we clear the stream.
Implemented timeouts for WaitForSingleObject.
Added assertions for functions not implemented.
Thread timed join now only available with _GUN_SOURCE.