Added an option to invert vertex alpha (mainly for Crocotile)

I was poking around with Crocotile models today, and it turns out that it writes vertex alpha with the opposite value that you’re supposed to, because this is the OBJ format and we don’t believe in standardizing.

So I added a button to invert the vertex alpha for mesh objects. If you try to import a file and see the wireframe is drawn but the mesh itself isn’t, try that button.