Should we add <input type=location rel=pixel> to MapML?

[prushforth]

In Testbed 15, the issue arose that in order to use an early implementation of WFS 3 as a surrogate for GetFeatureInfo from WMS, we could serialize a veeeery small bounding box under the user's cursor when they click on the map to trigger a query.

There being an <input rel=image (the default if not specified)> and an <image rel=tile> already, it seemed reasonable and possibly useful (outside of this use case) to add a rel=pixel possibility.

I added an implementation in GeoServer (and in the process I discovered that I had omitted to include rel="tile" in appropriate places, which I corrected in both the server and bundled client).

What do you think? I could remove the logic if it seems too thin.