Rendering variable elevation water surfaces [FEATURE]

[collinjroland]

Describe the solution you'd like
To visualize riverine flooding It would be very useful to be able to render inundation of non-uniform (i.e. sloping) water surfaces. Some users have sort of faked a uniformly sloping water surface by tilting the DEM (https://github.com/DOI-USGS/vizlab-chart-challenge-23/tree/main/07_hazards_conlon_colgin), but this does not work in all use cases. The solution I have roughly in mind would allow the user to provide a raster->matrix of water elevations (including NANs or something similar for areas without water) which would be used for the water rendering elevations. This is a great package and I really appreciate it's existing functionality!

Describe alternatives you've considered
I have considered trying to build a visualization using the Anuga viewer (https://en.wikipedia.org/wiki/ANUGA_Hydro), which I think includes this variable water surface elevation functionality, but that would require building Anuga model.