Removed the repeated allocations for VectorTable.

A new VectorTable is created every time GradientNoise3D is called. This _severely_ slows down noise generation.

I had recreated http://libnoise.sourceforge.net/examples/complexplanet/index.html and initial renders were taking more than 3 hours (I had given up and killed it at that point). Moving this so that it is only allocated once brings renders down to about 2 minutes.