ffmpeg: fix aspect ratio setting
This is done by adding a setsar filter at the beginning of the
This implementation is more robust, since does not modify the
filterchain description (which was creating potential syntax errors),
but directly modifies the filterchain structure.
This also changes the ffmpeg -aspect behavior, as following filters in
the filterchain can change the DAR/SAR set by the inserted setsar
Signed-off-by: Stefano Sabatini <email@example.com>