Fix Warning from Text Visual (OSX)

When running the Text example, it displays a warning twice (one for each visual):

````
python3.4 examples/basics/scene/text.py
WARNING: Program has unset variables: {'u_texh', 'u_step', 'u_texw'}
````

This patch includes the fix.

EDIT: This works on OSX but apparently the tests fail on Linux.  Cross platform issues?