Prefetch cells for collection view

[stevesoltys]

For iOS, the collection view currently does not provide a way to prefetch cells. This means that cells will load after the user sees them, rather than before.

https://developer.apple.com/documentation/uikit/uicollectionviewdatasourceprefetching/prefetching_collection_view_data

Untested on Android, not sure if there's something equivalent on that side of things.