Standardize API handling of ST_Simplify

[mem48]

Within the SQL we call St_Simplify(geometry, tolerance) to reduce the complexity of geometries, and thus reduce the amount of data sent to the browsers. Currently, each layer handles this independently and differently.

I suggest a standardised approach based on this table:

Zoom	Simplify
11	0.00412
12	0.00206
13	0.00103
14	0.00052

This should ensure that at any zoom level the geometries do not drift more than about 10 pixels for their expected location. Which is just about noticeable without causing significant changes at each zoom level.

[Robinlovelace]

Sounds good.