Add memory usage to the Voxel demo

[aaronfranke]

I recently added this to the TPS demo's debug menu, so I figured I would add it to the Voxel demo too.

[volzhs]

isn't it better to use String.humanize_size(bytes)? just my 2 cents.

[aaronfranke]

humanize_size always outputs 4 digits, for this demo I think it's better to only display increments of one MiB.

But thanks for bringing that up, and I will likely use that in the future.

[volzhs]

Oh i see :)