factor out tile extent as constant

[incanus]

[mb12]

In addition to this, the factor 8.0 needs to be factored out to layer_extent/tileSize.

Once that is done, then performance on retina displays can be improved by 4x by setting tileSize for iPad Air/retina devices to a higher value. Currently for the same zoom level, it loads 4x the number of tiles compared to non-retina devices.
Since a tile represents a fixed physical region on earth, for Vector sources tileSize should be a function of device physical size not pixel density.

[incanus]

Dupe of #250, really.