[Amibe] stitch: bug fix in collapse and merge
[Amibe] stitch: use isMutable to detect already projected vertices

... instead of isManifold because VertexMerger can modify manifoldness
during the process
[Amibe] stitch: let getXXBorderEdge return edges from the exected group
[Amibe] stitch: do not create non-manifold edges in boundary-only mode
[Amibe] Mesh.popTriangles: do not rebuild adjacency