dvdplayer: Allow multithread decoding for hi10p content by default

This allows decoding of some hi10p material on e.g. AMD Fusion with
both cores at the max. This introduces a new advancedsetting named
disablehi10pmultithreading to disable hi10p decoded multithreaded.
Merge pull request #2064 from fritsch/xbmc-upstream

dvdplayer: Allow multithread decoding when software is used