Allow the rendering buffer to be computed automatically

The new buffer option in streaming renderer requires the user to manually provide a buffer size.
In most common styles, it's possible to infer the optimal buffer size automatically, assuming stroke
sizes are literals (in general they can be constants).

If buffer is not set, this could be an automatic behaviour, that is, something not even requiring an extra parameter to be set, just parse the current layer style and set the extra buffer if needed (so do this in the queryLayer method).