Change rendering label natural=valley

[matthijsmelissen]

The following issue has been moved over from trac:

Two considerations on lines including natural=valley tag:

natural=valley has very limited usage (Openstreetmap currently lacks of information related to valley names) 

when using natural=valley in a line, the adopted font is very small (the same as streets); besides, the zoom visibility is limited to the highest ones. 

In order to provide a decent mapnik rendering for valleys and incentivate the usage of natural=valley, I suggest to adopt an automatically resizable font for the name tag with automatic character spacing; zoom visibility shall also be extended to additional levels.

This change could be implemented in two steps: the former could consist in a static font size (bigger than the one now used), a fixed (larger) character spacing and more allowed zoom levels, while the latter could introduce dinamic font size and dinamic character spacing according to the line length.

[matkoniecz]

It is outdated, natural=valley name rendering was from catch-all. Now it is not rendered.

[matkoniecz]

For now it is duplicate of #788