- removed "using" directive in .hpp
- reverse order of arguments for quaternion rotation, i.e. q.rot(v)
  instead of v.rot(q), where q is a quaterinon and v a vector
- pass the inverse of the view matrix to render_and_present_frame, to
  allow light calculation in shaders (we used to just pass the matrix of
  the quaternion of the transformation, i.e. discard scaling and
  translations)
- added another mesh and texture (viking_room) for testing purposes
- added transparent background option
- added Quaternion::look_towards(), which is the equivalent of
  Matrix4::look_at() but only for rotations
- various improvement to .obj parsing
- load texture coordinates from .obj file
- merged duplicate vertices in Mesh::linearize_indices()