lavfi/vf_aspect: improve compatibility of parsing.

Try to accept both the old "num:den" and the new "num/den"
syntax. Fix "aspect=2" being ignored due to a missing den.

Also remove a duplicated #include.
ffmpeg: make -aspect work with -vcodec copy.
Merge remote-tracking branch 'cigaes/master'

* cigaes/master:
  ffmpeg: make -aspect work with -vcodec copy.
  lavfi/vf_aspect: improve compatibility of parsing.

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