feature request: add slope to contour lines

[zerda-ocm]

On flat landscapes, contour lines often appear distracting and unhelpful. For example, when the elevation in an area varies randomly between 99 and 101, the resulting patterns can be wild and confusing. Embedding slope information within the contour lines could improve this. By toning down the appearance of contours in flat areas and emphasizing steep hills, a map could be more useful and visually appealing.