Add option to enable or disable Entity#offset

[xeolabs]

Breaking change

Entity#offset capability is now disabled by default, and needs to be enabled if we want to use it:

const myViewer = new Viewer(myCanvas, {
    enableOffsets: true
});

Not all applications use Entity#offset, so it makes sense not to impose that overhead on all applications.

See here for a demo of Entity#offset: https://xeokit.github.io/xeokit-sdk/examples/#BIMOffline_XKT_metadata_moveStoreys