AviSynth works on frame-based video by default, which can
be either progressive or interlaced. Some filters can break
frames into half-height fields, at which point it considers
the clip to be field-based (avs_is_field_based can be used
to check for this situation).

To properly detect the field order of a typical video clip,
the frame needs to have been weaved back together already,
so avs_is_field_based should actually report 'false' when
checked.
Signed-off-by: Stephen Hutchinson <qyot27@gmail.com>