fix tile caching

[ansis]

e1a7bba#commitcomment-4879825

The tile cache isn't properly used anyway; we should remove or refactor it. The tiles have to be reparsed in a webworker anyway, even if they are in cache, so the only thing we'd "save" is the redownload of the tile which is a non-issue anyway because the tiles are cached. Instead, we should profile the impact on memory usage and keep around the geometries in web workers for future use. This could go along with prefetching covering tiles (when zooming out).

[tmcw]

Grabbing this

[gundersen]

@tmcw @mourner how hard is fixing the tile cache for GL? It will be one of the biggest wins for me as I demo if the tiles stay cached in the browser so I can really show off the smoothness.

cc @edenh @rsudekum

[mourner]

@gundersen the vector tile downloads are already cached by the browser (for 2 hours), this ticket is only about the fact they are reparsed every time, which leaves some room for memory footprint / performance improvement, but not significantly noticeable.

If browser cache is not enough, we could add a cache manifest so that the tiles are cached forever (until reset).

I think the demo should be good to go in the current state, because any significant improvement would be non-trivial (not a last-minute fix). But maybe @ansis or @tmcw have suggestions.

[ansis]

Caching the parsed geometries and labels should have some noticeable impact. Label placement can take 200-400ms in some of the densest areas right now, so caching should let us skip redoing that every time a tile reappears on the screen.

[bsudekum]

Just to clear of some confusion around @gundersen's question. The tiles have to be reloaded no matter where they are coming whether they are cached or served. If you zoom around then turn off your wifi, the tiles will still load.

[ansis]

added in e52bce1 - It holds onto 20 most recently used tiles to avoid reparsing them when quickly switching between zoom levels.