Implement hillshading from HGT digital elevation model data. Similar to how it works in [Mapsforge](https://github.com/mapsforge/mapsforge) map library. - New `vtm-hillshading` module - See Android [HillshadingActivity](https://github.com/mapsforge/vtm/blob/master/vtm-android-example/src/org/oscim/android/test/HillshadingActivity.java) and desktop [MapsforgeTest](https://github.com/mapsforge/vtm/blob/master/vtm-playground/src/org/oscim/test/MapsforgeTest.java) <img src="https://github.com/user-attachments/assets/db27b587-ae3f-422a-9ee6-060935f86d2f" width="50%"/><img src="https://github.com/user-attachments/assets/42fe9ef1-b30b-4484-b3d8-06852bd6f24d" width="50%"/>
Implement hillshading from HGT digital elevation model data.
Similar to how it works in Mapsforge map library.
vtm-hillshadingmodule