Fix Grid regression where pinned columns were automatically un-pinn…

[ghsolomon]

…ed when the viewport became

too small to accommodate them.

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[ghsolomon]

For context:
https://www.ag-grid.com/react-data-grid/column-pinning/
https://www.ag-grid.com/react-data-grid/grid-options/#reference-columnPinning-processUnpinnedColumns