code: function mat3from4(out, mat4x4) {
      out[0][0] = mat4x4[0]
      out[0][1] = mat4x4[1]
      out[0][2] = mat4x4[2]
      
      out[1][0] = mat4x4[4]
      out[1][1] = mat4x4[5]
      out[1][2] = mat4x4[6]
  
      out[2][0] = mat4x4[8]
      out[2][1] = mat4x4[9]
      out[2][2] = mat4x4[10]
  }
docs: gets upper-left of a 4x4 matrix into a 3x3 of vectors