Different rendering by scale

[mrmaxguns]

Depending on the scale we choose for our map, we might want more or less information shown. Additionally, we might create a style that we want to reuse for other maps with different scales and extents.

Proposal:

The direct children of <layers> can be either layer types or the <scale> element. The <scale> element must have one or both of largest and smallest properties which represent the minimum and/or maximum scale at which the layer types within the <scale> element will be rendered. Note that the value of largest will actually be less than smallest because a large scale means "zoomed out":

<layers>
  <!-- Always rendered. -->
  <polyline ref="park" fill="#082d17"/>

  <!-- Only rendered if scale is more zoomed out than 1:10000 -->
  <scale smallest="10000">
    <polyline ref="highway" stroke="#2518ba" thickness="2"/>
  </scale>

  <!-- Only rendered if scale is between 1:1000 and 1:5000 -->
  <scale largest="1000" smallest="5000">
    <nodeshape ref="memorial" fill="#828c38" angle="0" radius="1" vertices="4"/>
  </scale>
</layers>