Add chunked support for texture shading

[rhugonnet]

The only exception is "texture_shading", I think. Because it's frequency-based, the attribute is not immediately consistent if computed in chunks, or on the full DEM extent.

Here's a plot:

@friedrichknuth Do I get the right understanding there? Do you see any way to address this?
I don't fully understand the underlying math. Is there a way to concatenate some "absolute information" per chunk, and later apply some kind of "scaling" for each chunk, eventually reconstructing back the full texture shading for the full DEM?
If yes, we'd have to add some logic specific to chunked texture shading, but it would be doable! 🙂

Originally posted by @rhugonnet in #845 (comment)