Add custom @2x handling for mapbox v4 tile API when tileSize: 512

[yhahn]

The v4 mapbox tile API supports 512x512 image tiles only when @2x is appended to the tile URL. If tileSize: 512 is specified for a Mapbox raster source force the @2x suffix even if a non hidpi device.

We can revisit this with the next version of the tile API which may look to provide 512x512 tiles to non-hidpi devices.

cc @kkaefer

[jfirebaugh]

@yhahn Can you elaborate on the motivation here?

If tileSize: 512 is specified for a Mapbox raster source

My understand is that this shouldn't happen / you shouldn't do this. You should always specify tileSize: 256 for a Mapbox raster source.

[yhahn]

You should always specify tileSize: 256 for a Mapbox raster source.

Yeah, this is not necessarily revising this assumption but at least allowing us and others to start testing more realistically what using tileSize: 512 would be like.

All newer raster imagery on the mapbox API (default and custom uploaded imagery via upload API) now has 512x512 hires versions. At the moment there's no way to make use of this in conjunction with the 512x512 tile rendering size in either GL js or native -- if you want to try this out, you're stuck stretching 256x256 tiles out.

In the future we'll need to support 512x512@1x and 1024x1024@2x tiles from the Mapbox API to make the tileSize: 512 switch fully.

[yhahn]

Adjusted approach after discussion: #1971