Makes it possible to use deinterlacers which output one frame for each field as fallback if field
matching fails (combmatch=full).

Currently, the documented example with fallback on a post-deinterlacer will only work in case the
deinterlacer outputs one frame per first field (as yadif=mode=0). The reason for that is that
fieldmatch will attempt to match the second field regardless of whether it recognizes the end
result is still interlaced. This produces garbled output with for example mixed telecined 24fps and
60i content combined with a field-based deinterlaced such as yadif=mode=1.
This patch orders fieldmatch to revert to using the second field of the current frame in case the
end result is still interlaced and a post-deinterlacer is assumed to be used.
Signed-off-by: lovesyk <lovesyk@users.noreply.github.com>