Fix: Make watermark distance independent of DPI value
When changing the DPI value from default (250) to a smaller one (e.g. 150), the texts in the watermark template went further apart from each other. This can be explained by the fact that the distance was 500. When rasterizing with a smaller DPI value, the distance stays fixed at 500. With the proposed change, the watermark texts have always the same distance, independent of the DPI value.