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.