Filter xstack
  Stack video inputs into custom layout.
    slice threading supported
    Inputs:
        dynamic (depending on the options)
    Outputs:
       #0: default (video)
xstack AVOptions:
  inputs            <int>        ..FV...... set number of inputs (from 2 to INT_MAX) (default 2)
  layout            <string>     ..FV...... set custom layout
  shortest          <boolean>    ..FV...... force termination when the shortest input terminates (default false)
  fill              <string>     ..FV...... set the color for unused pixels (default "none")


