Draw contour labels multiple times, using a configurable distance between labels about libosmscout

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Implemented for Qt and cairo (push pending), will now look at agg.

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This a simple backend problem. The MapPainter base class that handles the contour, the label and the style to the backend. The backend is free to implement any algorithm it finds suitable.

The backend (if look at MapPainetrQt for example) already knows the length of the path and the length of the label. It just has correctly implement the initial offset and the repeated drawing of the label with gap space until the end of the path is reached.

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Also implemented for agg backend. For the iOSX status/fix I have to ask the mailing list, because I do not have a Mac.

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Also implemented for iOSX. Closing.