Non-manifold input to SimplifyTopology in Boolean.Close test

[elalish]

First noticed in #171, but subsequently the same failure was found on master when running CUDA. The infinite loop is a byproduct of the mesh not being manifold. Technically IsManifold() checks two different things: first, is the data structure sane, and second are there any duplicated edges (which make it not a 2-manifold - two triangles attached to every edge). SimplifyTopology is designed to fix meshes that are not 2-manifold into ones that are 2-manifold. However, we should not be producing any intermediate mesh that isn't sane. I need to add an assert about this to make these problems easier to spot (this is the second time I've run into something like this).