Allow to generate mesh using separate thread process

[Toshiwoz]

See, this documentation to know how to do it, shouldn't be too difficult: http://docs.godotengine.org/en/3.0/classes/class_thread.html

[Toshiwoz]

I also partially implemented this, but it seems that threads never end, so I just found a workaround that is create a new thread every time, obviously not the right solution. It also seems to be slower, I'll add a property that will allow the user to choose.