Add more complex "bounding boxes" for tasks

[NotStirred]

All bounding box queries need to be extremely fast, as they will be per-cube for cube operations, and per-block for block operations

examples:

• placing a sphere, sphere distance function
• placing a hollow sphere. sphere distance function with smaller sphere distance function for negation within it

I now also realise that it's impossible to guarantee intersecting a cube with a block distance function will work without checking every block, so there may need to be an implementation for both