Multiple imagery backgrounds / imagery backgrounds as layers

[emi420]

Currently you can have one background and one overlay (raster over the background). It would be really good to have the possibility of adding multiple backgrounds and with functionality to hide/show them, so we can create maps with pre/post disaster imagery for example,. or to show how an area is changing with time (for monitoring things like deforestation, glaciar contractions, etc)

[emi420]

This is an example on why we would like to have the option of multiple backgrounds/overlays that can be turned on and off.

For doing the comparison between before and after a landslide:

https://umap.hotosm.org/en/map/mapeo-granizal-10julio_377

[emi420]

There's an issue in the uMap project that could be supported for this:

umap-project/umap#1814