[WIP] 3D Textures

Implementation of 3D textures using PyOpenGL.
With either gloo.clear or gloo.set_state, I get the following.

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Traceback (most recent call last):
  File "animate-images-slice.py", line 99, in on_initialize
    gloo.set_state(clear_color = 'white')
  File "/Users/irwin/Library/Python/2.7/lib/python/site-packages/vispy-0.2.1-py2.7.egg/vispy/gloo/wrappers.py", line 526, in set_state
    globals()['set_' + s](*args)
  File "/Users/irwin/Library/Python/2.7/lib/python/site-packages/vispy-0.2.1-py2.7.egg/vispy/gloo/wrappers.py", line 206, in set_clear_color
    gl.glClearColor(*_check_color(color))
  File "/Users/irwin/Library/Python/2.7/lib/python/site-packages/vispy-0.2.1-py2.7.egg/vispy/gloo/wrappers.py", line 62, in _check_color
    color = np.array(color, float)
ValueError: could not convert string to float: white
WARNING: Error invoking callback for event: <Event blocked=False handled=False native=None source=<Vispy canvas (PyQt4 (qt) backend) at 0x109d88e90> sources=[<Vispy canvas (PyQt4 (qt) backend) at 0x109d88e90>] type=initialize>