Using per-size arenas behaves like a memory leak when
the user creates many VoxelBuffers of random sizes repeatedly.
Now memory blocks of the next power of two are used instead.
VoxelBuffers with power-of-two size will fit best, while also being
the most common. Non-power-of-two will use a bit more memory,
but such buffers are often temporary and less numerous.
- New importer, needs restart to switch but it's a limitation of Godot
- Might blow up if the scene spans a really big area, may be optimized in the future.
- The scene importer remains the default choice
- Promoted some ints to 64-bit to avoid oveflowing with large volumes or distances
- Handle a few cases of malloc failure
