Filter vstack
  Stack video inputs vertically.
    Inputs:
        dynamic (depending on the options)
    Outputs:
       #0: default (video)
vstack AVOptions:
  inputs            <int>        ..FV.... set number of inputs (from 2 to INT_MAX) (default 2)
  shortest          <boolean>    ..FV.... force termination when the shortest input terminates (default false)


