Fix min/max position not being correct due to multithreading

[CCInc]

The code was in place to handle this:

			// merge min/max of this batch into global min/max
			for (int i = 0; i < outputAttributesCopy.list.size(); i++)
			{
				Attribute &source = outputAttributesCopy.list[i];
				Attribute &target = outputAttributes.list[i];

				lock_guard<mutex> lock(mtx_attributes);
				target.min.x = std::min(target.min.x, source.min.x);
				target.min.y = std::min(target.min.y, source.min.y);
				target.min.z = std::min(target.min.z, source.min.z);

				target.max.x = std::max(target.max.x, source.max.x);
				target.max.y = std::max(target.max.y, source.max.y);
				target.max.z = std::max(target.max.z, source.max.z);
			}

However, outputAttributesCopy min/max was never being set; instead, outputAttributes min/max we being assigned to directly. As a result, the code above did not do anything, and outputAttributes got improperly assigned/overriden by multiple threads.

[m-schuetz]

This is specifically for the position attribute, which doesn't get an attribute handler that populates the copy as it should, right?
Your PR seems to make sense. Thanks!