Manifold Decompose doesn't preserve vertex properties about manifold

[harmanpa]

It turned out that the only cause of this was that meshRelation_.numProp was not set in Manifold::Impl::GatherFaces while the vertProperties and properties were. However I've added a test and associated test utility (PR incoming) to check the properties in a resulting mesh match those set via SetProperties.

[pca006132]

Probably a bug because decompose was there before vertex properties.

[harmanpa]

I'd be happy to try to implement and test a solution, do you know of anywhere in the code that also filters the vertex properties by index that might be similar @pca006132 ?

[elalish]

That would be excellent, thanks! It might help to look at CompactProps.

If you get excited about cleaning up Decompose, there's another issue open about it: #745 (comment). Ideally it shouldn't take too much to make Manifold::Decompose act more like CrossSection::Decompose