Improve 'perceived performance' by preloading lower zoom level tiles

Nobody has claimed this yet.

A number of my users have complained that things "feel" slow and "blocky" when panning around. Basically, as they drag-pan around on their phones, they see a lot of white blocky tiles being loaded. Much more so than google maps.

Initially, I thought I should try and increase the viewport. Not sure about the terminology here, but basically make sure more neighbouring tiles are preloaded. However, I think that quickly leads to 'how long is a piece of string'. And after a bit more digging, I think there is a better and easier solution to this perceived performance.

Rather than loading more neighbouring tiles at the current zoom level, we could preload (and render!) the lower zoom level tiles. That way, when drag-panning, you would not see any white unloaded tiles. Instead you'd see details from the lower zoom level, which would gradually improve quality as the tiles at the current zoom level are loaded.

So if we are at zoom level 12, we could/should load the tile at those coordinates for level 11 (and maybe even 10, 9, etc - so that if the user does a quick zoom out pinch gesture, they still dont see unrendered tiles).

This should make a significant improvement on 'perceived' performance.

No files, tests, or entry points are named. Start by locating the tile loading and rendering paths, then determine how lower zoom levels could be preloaded and displayed during panning and zooming. Done should mean that lower-resolution tiles appear instead of white unloaded areas while current-resolution tiles load.

Written by the indexing model from the issue text.