VoxelGI fixed rendering outside of non-uniform bounds

[Robert-K]

Closes #62930

Before:

After:

What was the problem?

The GPU receives the VoxelGI transform in cell space for quicker tracing.
The bounds need to be in cell space, too, so the octree_size was passed.
That's incorrect, since for non-uniform bounds, it doesn't match the actual bounds in cell space.

Instead, we scale the bounds using the basis of the to_cell transform and pass that.
We still have to map back to world units, hence we need to pass the octree_size as well (one could also pass the cell size (1.0/octree_size) instead for further optimization)...
Conveniently, a vec3 already existed as padding in the VoxelGIData struct passed to the GPU, so I just turned that into the octree_size.