Add multiple viewports to mesh importer

[adamkewley]

Estimate: 4-6d

It's a nice feature for advanced users, but would require a fair bit of development. During UX sessions, almost no users have used more than one viewport (although, again, I imagine power users will).

The mesh importer currently only supports one viewport because it was initially developed as a quick one-off screen that's mutated into a full-blown UI :wink: - it now needs multi-viewport support.