Fix the performance issues when you scroll deep into the Photos Library

[alexwlchan]

If you press ? to get a random unreviewed photo, the app massively slows down – I think it's pulling thousands of thumbnails into memory at once as it renders the intermediate LazyHStack. It feels like this is fixable, if I can stop it actually fetching the assets until they're shown on screen.

[alexwlchan]

I tried replacing the thumbnail images with static images, and I still see the latency – so it's not retrieving the image pixels that's the issue.

I wonder if the issue is inside PHAssetHStack – when you scroll, it's presumably making thousands of requests for a PHAsset from the Photos Library, even ones which aren't on screen.