render mud from z10

continuation of #420, fixes #1018 
> http://www.openstreetmap.org/#map=13/53.6174/0.0715

> Here there is both mud and sand areas adjacent to each other. At z10, 11 & 12 only the sand is rendered, creating an 'island' of sand. In reality this is a single, continuous tidal flat.

Also, with smaller areas of mud rendering is OK http://www.openstreetmap.org/#map=10/53.6918/9.3082 http://overpass-turbo.eu/s/5oY

![selection_005](https://cloud.githubusercontent.com/assets/899988/4594847/4dba426e-5093-11e4-9572-b53cc744283f.png)