Add zoomOffset and tileSize options

[saesh]

LeafletJS offers an option for an offset which is applied to the zoom level given in the URL. This is helpful when big tile sizes are used and the zoom level needs to be adjusted for the increased tile size. Bigger tile sizes can be beneficial to decrease the amount of requests to fetch all map tiles.

From the Mapbox documentation on 512px tiles:

Note that 512×512 image tiles are offset by one zoom level compared to 256×256 tiles. For example, 512×512 tiles at zoom level 4 are equivalent to 256×256 tiles at zoom level 5.

When bigger tile sizes are used LTileLayer has to be configured as well using the tileSize prop.

A setting of tileSize=512 and zoomOffset=-1 will result in an identical map image to using tileSize=256 and zoomOffset=0 but with a quarter of tile requests.

[linusg]

Please keep the options alphabetically sorted in the docs & config.

[linusg]

Please, instead of adding another commit to fix the previous ones, amend them so they're correct in the first place. Let's aim for a clean commit history. :)

I.e. one commit adding the zoomOffset option + docs, and one doing the same for tileSize.

[linusg]

Thanks!

[linusg]

Released in v2.12.0 just now, sorry it took so long!