[FFmpeg-devel] [PATCH] Populate field order returned by avs script, Trac Ticket 8757

[emcodem at ffastrans.com]

The purpose of this is to tell ffmpeg which field order the returned 
clip of the avisynth .avs script decided to finally deliver, which is 
handy in an automated environment.
Interlacing is generally a very hard topic in avisynth, the huge comment 
is neccessary to explain my reasons.
Additionally to the comment in the patch, i can tell that i set unknown 
instead of progressive for backward compatibility. (i was struggeling 
with this, maybe i should have even left it unset in case it is not 
clearly tff or bff interlaced.

    /*  Set interlacing type. 
http://avisynth.nl/index.php/Interlaced_fieldbased
     *   The following typically only works when assumetff (-bff) and 
assumefieldbased is used in the avs.
     *   This is because most avisynth source plugins do not set the 
parity info in the clip properties.
     *   We could use GetParity() to be more accurate but it decodes a 
frame which is
     *   expensive and can potentially lead to unforeseen behaviour when 
seeking later.
     *   In case Parity is not known, we still cannot guarantee that
     */

Tests with different avisynth source plugins, e.g. directshowsource, 
ffms2, mpeg2source delivered the expected result.
Make FATE passed.
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