lavu/rational: add syntactic sugar.

Add a function to create a rational
and macros for common values.
lavfi/af_amerge: use av_make_q.
lavfi/avf_concat: use av_make_q.
lavfi/vf_tile: use av_make_q.
lavfi/dualinput: fix shortest option.

Fix trac ticket #3315.
Merge remote-tracking branch 'cigaes/master'

* cigaes/master:
  lavfi/dualinput: fix shortest option.
  lavfi/vf_tile: use av_make_q.
  lavfi/avf_concat: use av_make_q.
  lavfi/af_amerge: use av_make_q.
  lavu/rational: add syntactic sugar.

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