UMVE: Added support for the per-vertex value attribute in the mesh list
DMRECON: share image pyramids between threads

Threads reconstructing views from the same scene simultaneously can
share some of their image pyramids when they both use the same
neighboring view. Store them in a global cache.
MVE: Options struct for save_ply_file instead of parameter list
Merge pull request #38 from Waldteufel/cache

cache image pyramids
Merge branch 'master' of github.com:simonfuhrmann/mve