Fix dash array discontinuities across tiles

[aaime]

Currently there are visible discontinuities in dash array across tiles. osm-bright solves that by disallowing line clipping, however this would cause OOM in the default JVM (probably not with Marlin, to be verified).
We'd need some smart clipping that walks the perimeter of the line instead, and cuts outside the view, at a integer number of pattern cycles from the beginning of the line. This is certainly expensive, so to be activated using a vendor option.