dmrecon: don't crash on missing view

Previously, dmrecon crashed if the bundle file contained a view id
larger than the maximum view id in the scene.
MVE: load embeddings in a thread-safe way

In some places, cache_cleanup could remove an embedding after it was
loaded with ensure_embedding but before it was actually used.
Merge pull request #54 from Waldteufel/dmrecon-crash

fix null pointer crashes regarding missing views/embeddings