Added merge for LOD, changed LOD parser
Now it possible to merge multiple objects and use LODs together. All
materials should be present in each LOD, in this way each geometry has
the same range of LODs, otherwise, if some LOD is missing the following
LOD will extend its range to lower distances and it can overlap other
geometries.
Revised all the LOD creation, instead of a bool it uses a set() to keep
track of what geometries are updated with the last LOD.
On the LOD object names, it is no more necessary for the distance to be
right aligned; also, if an object name is missing the _LOD tag, it is
considered as the first LOD with distance 0.
Added an option for keeping the materials separate in different geometries
while merging objects.
Keep separate object name and Blender object name.
Added 30° limit for normals in not strict LODs
A not strict LOD will try to find the most similar vertex, but it was
even adding vertex with flipped normals.
Now there is a limit of 30° between normals, if no vertex is found a
new one is added (a warning signals it).