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 #197 (comment)

[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)

[jhotadhari]

I copied lots of mapsforge code and build a HillshadingTileSource

First idea. Maybe some improvements and it could be implemented in vtm.

[demiantres]

I copied lots of mapsforge code and build a HillshadingTileSource

First idea. Maybe some improvements and it could be implemented in vtm.

Nice work. I was also considering to leverage OpenGL ES shaders for hill shading but I have not made progress yet.

[devemux86]

OpenGL has many problems, rendering artifacts and even fails to run.

Mapsforge is the recommended map library, now with map rotation and new hill shading:

• Allow for free rotation of the map mapsforge#7 (comment)
• Hillshading new algorithms and improvements mapsforge#1551