mesh id handling

[pca006132]

See #678 for discussion. Basically:

1. Use zero as a special mesh ID. IncrementMeshIDs will only remap non-zero IDs. Constructors automatically assign mesh ID zero. This allows coplanar triangles to be collapsed by default unless explicitly opt-out.
2. Allow AsOriginal(id) to take an optional parameter to specify the ID to apply to. This allows preserving the ID zero.