Don't use boundary=protected_area for landcover coloring

[almccon]

The landcover colors in this area are strange-looking shapes that don't reflect the true landcover: http://maps.stamen.com/terrain/#10/37.9338/-116.2923

The reason is that the green color is caused by some boundary=protected_area features, specifically a bunch of Wilderness Study Areas: http://www.openstreetmap.org/way/398258173

There are natural=wood features in the same area that are more appropriate: http://www.openstreetmap.org/way/178377502

Especially as the tagging in the US is trying to separate the National Forest features from the actual landcover mapping, we should avoid deriving any landuse colors based on administrative features like boundary=protected_area.

[almccon]

However, I think we still need to use leisure=park as a landcover type, because it will help color in areas in urban regions.

I'm on the fence about leisure=nature_reserve, for example http://www.openstreetmap.org/way/371949411