Re-use diff.vSizeMM result
Re-use diff.vSizeMM result

At first I thought, well we don't need vSizeMM here; we can just use vSize2 which is cheaper. But then we are actually needing vSizeMM down lower anyway, so I'll not change the workings of the algorithm but we can at least prevent having to re-calculate it every time. Might make it a little bit faster, although a compiler would probably do this for us anyway since it may be able to see that the vector doesn't change in between. But then again, it may only be able to see that now that we have made diff const.

Ultimaker/CuraEngine,Ultimaker/CuraEngine