This adds support for background loading of data from vector layers into 3D map views. Until now, loading (and tessellation) would freeze the GUI completely - this could take many seconds depending on the complexity of input data.
The basic vector layer renderer and rule-based renderer were converted to use QgsChunkedEntity which is already used for terrain rendering. There are two more improvements in addition to unlocking of GUI: - loading process is multi-threaded instead of using just a single core - loading is done in tiles - so it is possible to see the tiles with 3D data appearing while other data are still being loaded
There is a new configuration option in 3D tab of vector layers - it determines how deep will be the quadtree. For example, one zoom level means there will be a single tile for the whole layer. Three zoom levels means there will be 16 tiles at the leaf level (every extra zoom level multiplies that by 4, so I have limited GUI to max. 8 levels which gives ~16K tiles which is already a lot).
How a vector layer's tiling quadtree gets populated: all internal tree nodes are empty and thus the 3D map scene tries to immediately replace them with their children - this goes until leaf nodes are reached. Only nodes at the leaf level currently hold any data. This may change in the feature when we introduce more elaborate strategies - for example, internal nodes may contain a small percentage of features of the child nodes (this would allow us to show something while zoomed out a lot, not requiring to load all data).
For debugging purposes, there is also a new configuration option "show bounding boxes". This allows you to see bounding box of each tile (especially useful if there are some issues with tiles not showing up when they should).