viewer: fix on contexts with 16bit depth buffer.
viewer: fix on contexts with 16bit depth buffer.

The depth range was way too much for that. Thanks a lot to @Lectem for
reporting and guiding me through this.

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