Implement min/max native zoom for templated image layers

[ben-lu-uw]

*and templated feature layers. Currently the attributes min and max on <map-input type="zoom"> (min/max native zoom) are not performing any function. On templated tile layers when the native zoom is left, tiles are scaled till the zoom bounds set in the meta.

[prushforth]

An image layer (WMS) can be accessed without having a zoom input at all, because the pixel width/height as well as the PCRS extents of the viewport are required parameters, so the scale can be calculated on the server with that information.

That doesn't mean that it shouldn't be possible to constrain the requests by providing a <meta name="zoom" content="...min=...,max=...,"> <- maybe need to add ability to include a "step=" option here...