avfilter/vf_tinterlace: Fix vf_tinterlace mode 6 (interlacex2)

The purpose of this filter mode is to allow interlaced content to
display properly in interlaced video modes, as described in
http://forum.xbmc.org/showthread.php?tid=81834 and
https://github.com/mpv-player/mpv/issues/624#issuecomment-37685195 . The
filter doubles the video frame rate, but does not work properly because:
(1) it does not set the properties of the output stream to indicate the
doubled frame rate, and
(2) it does not set an appropriate PTS on the extra frames.
The attached patch fixes these problems by settling these values the
same way they are set in vf_yadif mode 1 (field) which also doubles the
frame rate.

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>