Split assimp model importing into thread-safe IO and OpenGL object creation

The assimp importer now allows threads without an OpenGL context to load assimp::Importer::MeshData from a file on disk, which can then later be used to create the corresponding SharedVertexArray.

Additional changes:

• Added a new property to control the aiProcess_FlipUVs assimp postprocessing flag
• Moved core logic to static methods for easier access across threads

MeshData is a data-heavy type that is passed per-value. In GLOW we usually use shared_ptr's for this (SharedMeshData).

I added this feature in 7c65f00e. Thanks for the ground-work though!

closed