erdblick only queries and visualizes tiles that are in Cesium viewport

[l-laura]

For on-demand data retrieval from mapget, we need to calculate the tile IDs of the Cesium viewport. core/src/aabb.cpp implementations are there to help.

Upon initialization or a change in the viewport, erdblick should:

1. cancel any previous ongoing mapget requests,
2. determine which tiles in the viewport are missing, and query mapget to retrieve them,
3. determine visualised tiles which are not visible anymore and remove them.