Be able to set buffer-size at layer level

[dgaubert]

With mapnik we can set buffer-size at layer level:

<Layer name="wadus" srs="+init=epsg:3857" buffer-size="64">

If we try to set buffer-size at layer level directly in CartoCSS:

#layer { buffer-size: 64 }

then carto raises the following error:

Error: Map properties are not permitted in other rules

In order to take advantage of mapnik's feature we need to be able to set buffer-size as layer's property as we can do at Map level.

cc @rochoa, @javisantana, @jorgesancha

[javisantana]

what reference are you using?

If we can use the Map {} buffe-size I think is an low priority issue

[rochoa]

To be clear: this is a nice to have, we don't need it for the migration.