fixes fransform from carthesian to spherical coordinates

Steps to reproduce:

Open `Empty.html`

``` 
mathbox.viewport({
  type: 'sphere',
  range: [[-π, π], [-π/2, π/2], [-1, 1]], // Range in X, Y, Z (Longitude, Latitude, Radius)
  sphere: 1,                              // Morph between cartesian (0) and spherical (1)
  scale: [1, 1, 1],                       // Scale in X, Y, Z
  rotation: [0, 0, 0],                    // Viewport rotation in Euler angles
  position: [0, 0, 0],                    // Viewport position in XYZ
}).vector({data:[[0,0,0],[1,2,0.5]]})
```
Arrow and line will be misaligned.