Make the passing of the camera position to the ChunkMeshUpdateManager more robust and clear.

This pull requests adds one patch to #1435: Instead of passing a camera position updates implictly via a Vector3f, the camera gets passed explictly and in a way that ensures that the change is visible to the other thread.

It's not the best solution, but better than relying on the game to use always the same camera position vector object.

I think @MarcinSc's improvement is very nice and I hope that it makes it to main develop branch with this patch.