Added deduplicating vertices to the stl loader.

After two unsuccessful and unmerged branches, I finally did something useful on main.
I used to deduplicate the vertices in stl_reader and then reduplicate them in the loader. But now, we just dedup in stl_reader and use those. That also allowed me to enable ambient occlusion, which was disabled for stl before. And now crease smoothing fully works too. And it’s faster (runtime wise, load time is slower because of ambient occlusion and crease smoothing that weren’t there before).
So an all-round very positive change, and also an end to my non-merging dry spell.