Fixing PowerOfTwo SpriteFont crash on iOS

Fixed a bug in the SpriteFont processor. For iOS the underlying texture has to be power of two and square, the content processor only checked if the texture was square.  This bug has been reported in the past, and people have been avoiding it by just playing with fonts and font sizes.  @see http://monogame.codeplex.com/discussions/404006 for an instance of how the bug shows itself when running on iOS.

Make sure when reproducing the problem you have to be running with a debug build of monogame or the opengl error wont be checked for.