Normals are not defined for travel moves, so we need to enable the array
after drawing them, not before

This is probably a good call for the future... Changing lights in
glpanel/gcview won't break stlview anymore

(and fix viewport scaling for non square viewers)

This way we can easily yield at the end of each iteration. The remaining
issue is mostly how to handle the costly numpy.fromiter, which we don't
want to do uselessly several times.

But in my world, you have nothing to fear.