viewer: make the example usable on OpenGL ES.
viewer: make the example usable on OpenGL ES.

We do have `ports` branch for ES ports, but this is such small change
that it won't hurt if this is exposed to the users in the example
documentation.

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