[rig] fixed bug for vMainPoses
[rig] fixed bug for vMainPoses

poparteu/openMVG#145

vMainPoses was a vector containing only the valid main camera poses and
yet it was accessed as if it were the full vector with the invalid
poses.

Now it is a map so that it can be still accessed with the full vector
indices.

poparteu/openMVG,poparteu/openMVG,poparteu/openMVG,poparteu/openMVG