viewer: use Vector3::angle() instead of homemade solution.
viewer: use Vector3::angle() instead of homemade solution.

Squareys/magnum-examples,mosra/magnum-examples,Squareys/magnum-examples,mosra/magnum-examples,Squareys/magnum-examples,mosra/magnum-examples,mosra/magnum-examples,Squareys/magnum-examples,mosra/magnum-examples,mosra/magnum-examples,mosra/magnum-examples