WIP tris selection

cc @Eric89GXL @dgwakeman @agramfort 

will share a validation example soon.
That seems reasonable. Have you tried other labels to see if it's consistently off by 3? My guess is that it will usually be off by a little, but not by the same amount every time. That would be consistent with labels having some vertices that aren't part of any triangle whose other two vertices are also in the label, like in the example I tried to draw above `<|>-`.