VSFilter: Improve the external subtitles naming.

ISR: Reuse subtitle name on reload.
BD: Open index.bdmv directly by internal LAV and let it select playlist.
Fix DXVA status for some decoders after 4a79186.

Checking status only once is not reliable.
Even LAV Video Decoder can disable HW decoding on fly except DXVA Native mode.
Show HW decoder name in status bar.