Add rendering for area:highway on z19(+)

[kocio-pl]

We should start rendering some notable microscale elements, such as area:highway. Taginfo says now we have 14.342 such objects worldwide - it's not as much as popular highway keys, but more than a raceway we already render and it's a visible number in itself. Rendering should start from z>=19 (or maybe z>=18?), because this zoom level really shows the problem with linear highway rendering.

[matthijsmelissen]

Duplicate with #180.

[dieterdreist]

Am 16.04.2015 um 01:04 schrieb kocio-pl notifications@github.com:

Rendering should start from z>=19 (or maybe z>=18?), because this zoom level really shows the problem with linear highway rendering.

I'm in favor of rendering them below (before) the key highway, from z17 and higher