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

Revert revision 13903. It is implemented in a different way by checking if the option --dshow-chroma is set. If it is set then the chroma is forced, otherwise it is not. This should solve the regression of previous commit, by letting users specify the chroma type to use. To get the previous default behaviour specify IV420 as preferred chroma type either on the commandline or in the Capture Device advanced tab.