Enable use of custom shaders with TextureFont.

Only bind a new shader if there isn't a texturing shader already bound.

Example gradient shader with TextureFont:
![gradient screenshot](https://cloud.githubusercontent.com/assets/81553/5092975/b86a21ae-6f21-11e4-8d3f-60808dc99e28.png)
In same project, using TextureFont without binding a shader first still works as expected:
![stock screenshot](https://cloud.githubusercontent.com/assets/81553/5092976/b86d68dc-6f21-11e4-8dd9-92e80d48cb3b.png)