Optimized distanceToCamera()

For testing against 200k nodes, new code saves almost 10 milliseconds over old code

testDistanceA() distanceSquareToPoint() took 13.145000 milliseconds
testDistanceB() distanceToCamera() took 14.162000 milliseconds
testDistanceC() distanceToPoint() took 15.010000 milliseconds
testDistanceD() oldDistanceToCamera() took 23.689000 milliseconds