title: interpolate between rotation matrices
body: <p>i have two rotation matrices that describe arbitrary rotations. (4x4 opengl compatible)</p>
  
  <p>now i want to interpolate between them, so that it follows a radial path from one rotation to the other. think of a camera on a tripod looking one way and then rotating.</p>
  
  <p>if i interpolate every component i get a squeezing result, so i think i need to interpolate only certain components of the matrix. but which ones?</p>