Explicit bounds extension

Rendering algorithms may need to make decisions based on object/scene bounds. For rasterization APIs near and far planes need to be selected. Shadow mapping requires setting up a shadow projection encompassing all shadow casters etc.

On the other hand scenes may contain large objects such as ground planes whose size does not necessarily correspond to the area of interest in the scene. In these situations it may be preferable to explicitly set the bounds instead of having them implicitly derived.

This extension would add a parameter `"bounds"` of type `FLOAT32_BOX3` to at least the world object but potentially also to instance, group, geometry and spatial field.

No file, test, or entry point is identified; start by reviewing issue #56's proposed FLOAT32_BOX3 bounds parameter and deciding which object types it covers. Done when the extension's scope and bounds behavior are specified for the selected objects.

Written by the indexing model from the issue text.