Issue 190: accept None for font name on Linux.
Issue 190: accept None for font name on Linux.

niklaskorz/pyglet,niklaskorz/pyglet,niklaskorz/pyglet,adamlwgriffiths/Pyglet,seeminglee/pyglet64,adamlwgriffiths/Pyglet,adamlwgriffiths/Pyglet,seeminglee/pyglet64,niklaskorz/pyglet,seeminglee/pyglet64,adamlwgriffiths/Pyglet