Filter hstack
  Stack video inputs horizontally.
    Inputs:
        dynamic (depending on the options)
    Outputs:
       #0: default (video)
hstack 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)


