This commit optimizes the node for the case where it works on many splines by allowing it to generate mesh data from their combinations in parallel. By itself, this made the node around twice as fast in my test file with a result of 20 million vertices, around 600ms instead of 1.2s before. That isn't actually a very good result; it reveals another bottleneck, a single threaded loop over all face corners in the mesh normal calculation code. As a simple change that might improve performance in some situations, this commit moves normal calculation out of this node, so at least the work isn't wasted if the mesh is changed later on in the node tree anyway.