mpeg: Add passing DVD navigation packets (startcode 0x1bf) to caller to allow better playback handling of DVDs.

The two types of packets (PCI and DSI) are passed untouched but combined by the new codec ID AV_CODEC_ID_DVD_NAV.
 The first 980 bytes in the packet contain the PCI data. The next 1018 are the DSI data.
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>