ffmpeg: init filtergraphs only after we have a frame on each input

ffmpeg: init filtergraphs only after we have a frame on each input

This makes sure the actual stream parameters are used, which is
important mainly for hardware decoding+filtering cases, which would
previously require various weird workarounds to handle the fact that a
fake software graph has to be constructed, but never used.
This should also improve behaviour in rare cases where
avformat_find_stream_info() does not provide accurate information.

This merges Libav commit a3a0230. It was previously skipped.

The code in flush_encoders() which sets up a "fake" format wasn't in
Libav. I'm not sure if it's a good idea, but it tends to give
behavior closer to the old one in certain corner cases.

The vp8-size-change gives different result, because now the size of
the first frame is used. libavformat reported the size of the largest
frame for some reason.

The exr tests now use the sample aspect ratio of the first frame. For
some reason libavformat determines 0/1 as aspect ratio, while the
decoder returns the correct one.

The ffm and mxf tests change the field_order values. I'm assuming
another libavformat/decoding mismatch.

Signed-off-by: wm4 <nfxjfg@googlemail.com>