Support HGT for offline hillshading

[ztNFny]

Unlike mapsforge vtm currently supports only online-sources for hillshading (e.g. MapilionMvtActivity.java). It'd be nice to have support for HGT format hillshading information to enable offline hillshading as well

VTM does not support currently Hillshading from digital elevation model data like HGT format. That works in Mapsforge with a different engine, processing bitmap images (mapsforge/mapsforge#923).

Originally posted by @devemux86 in https://github.com/mapsforge/vtm/issues/197#issuecomment-422341553

[devemux86]

It could work with a new offline bitmap tile source, with bitmap tiles generated from the hillshading data. (code exists in Mapsforge)

This can be combined with any existing map source, like when using online hillshading bitmap tile source. (like the online hillshading example)