Previously the result of the CAPS query and ACCEPT_CAPS depended on what kind
of caps were last set, and e.g. if we last had interlaced caps or not. That's
just broken.

Also previously the handling of non-sysmem caps features was rather random and
unusuable.

Now the behaviour is the following, depending on the mode property:
1) mode=disabled
  Completely do passthrough of everything
2) mode=interlaced
  Only accept formats we can actually deinterlace, and accept interlaced
  and progressive content and always run the deinterlacer and output
  progressive content
3) mode=auto (i.e. playbin)
  Accept all progressive formats as passthrough, accept all formats that we
  can deinterlace ourselves (which we do then), but also accept everything
  else for which we then just passthrough. In auto mode, deinterlacing is best
  effort: If we can, we deinterlace, if we can't we just output interlaced
  content.

https://bugzilla.gnome.org/show_bug.cgi?id=720388
https://bugzilla.gnome.org/show_bug.cgi?id=760553