Add a way to change the mesh scalar before a clip is loaded

[tim-depthkit]

Calling setMeshScalar before loading a clip causes an error, so the only other way to do it is by setting this.meshScalar directly, which is supposed to be a "private" member.

This is an issue because using the default mesh scalar may cause some devices (iPhone, iPad, etc) to run out of memory before you have the opportunity to set it to something lower.

We should either update setMeshScalar to not attempt to rebuild geometry before a clip has been loaded, or add some other way to specify the default mesh scalar, in the constructor for example.