dshow.cpp: Removed default preferred media types (I420) as it was reported by Paul Sokolovsky to cause kernel crashes
           with some buggy drivers. I added these media types a while back to get around another bug for drivers that
           didn't offer any preferred media types.
           Hopefully, the latter bug should be indirectly fixed when I added support for IAMStreamConfig.
           Moreover, I modified the selection of a default format in IAMStreamConfig to take
           GetFourCCPriority into a account.

* add support for negative timestamps
* add support for the delay parameter
* add support for reading the DVD palette of vobsubs
* free the used memory when we close the vobsubs

To use it, put a string in p_object->psz_header, all messages from this object and its parent will have the header prepended