Add mapml css rules to deal with clipping of vector tiles' content

[prushforth]

The features are fully rendered as paths, but the tile is being clipped by the CSS rendering engine at its bounds. A workaround to that may be to let the tile render outside its 256x256 bounds (a bit).

See overflow-clip-margin for possible albeit non-cross-browser solution.

and

geosolutions-it/geoserver#363 (comment)