Add option verbose debugging to dynamic BVH.

[lawnjelly]

In order to test tree balancing, this prints out the tree after certain operations. This is enabled at compile time only, by setting the DYNAMICBVH_VERBOSE_DEBUG define at the top of the header.

Notes

• This can either be merged as is or used as an example for adding debugging, or we can keep debugging out of the class. However with trees of these sort it is usually useful to have some debugging support.
• This debugging shows the current BVH is not working correctly, see Replace Octree by DynamicBVH in cull code #44623

[lawnjelly]

Forgot to close this, I believe there are plans to port the bvh from godot 3.x to 4.0 so adding features to this version may no longer be desirable (we can always resurrect if plans change etc).