Currently, when the coordIndex attribute of an IndexedLineSet contains a
polyline with > 2 indices, X3DGeoHelper::coordIdx_str2faces_arr() will
incorrectly determine the primitive type to be aiPrimitiveType_TRIANGLE or
aiPrimitiveType_POLYGON instead of aiPrimitiveType_LINE.
To fix this, this commit adds functions to explicitly handle an IndexedLineSet.
Fixes#3101
Several places in the code call `std::vector<aiVector3D>.emplace_back(0, 0, 0)`. The constructor of `aiVector3D` actually expects arguments of the type `ai_real`, (alias of `float` if compiling without `ASSIMP_DOUBLE_PRECISION`) but the literal `0` is of type `int`.
`emplace_back()` does support promotion, but `int` to `float` is a potentially lossy conversion. tl;dr: On warning level 4, MSVC spits out a very deeply nested `warning C4244: 'argument': conversion from '_Ty' to 'TReal', possible loss of data with _Ty=int and TReal=ai_real`.