Watermark on request

The current watermark option always merges the watermarkfile on top of the tiles and then saves the watermarked tile in the cache.
I've added a option which still let you see a watermark on requested tiles, but which will not be saved watermarked to the cache.

To get this behaviour you have to add following to the configuration file:
```xml
<tileset name="test">
    <watermark onrequest="true">/path/to/file</watermark>
</tileset>
```