Refactor MeshHelper

[isHarryh]

Summary

This PR does some improvements to MeshHelper.

1. Making flat_list_to_tuples an inline function.
2. Introduces the lists_to_tuples function to fix the type error that List[List] produced by the zeros function cannot be assigned to tuple-member-lists such as List[Tuple3f] List[Tuple4f].
3. Refactors the get_triangles function.
   1. Changes the way to generate triangles lists.
   2. Adds a indexCount fix for TriangleStrip submeshes, see AssetStudio's code.
   3. Removes the TODOs in the get_triangles function (I have tested that using NumPy as_strided function cannot realize a speed-up).