text is not properly clipped in 1.2.1

matplotlib 1.2.1, python 2.7.3, OS X 10.8.3, TkAgg backend

Calling text() to render and then interactively panning the image, the resulting text is not properly clipped to the data window, and over flows it.

Code to reproduce:
```python
import matplotlib.pyplot as plt

plt.ion()
fig = plt.figure()
ax = fig.add_subplot(111)

t1 = ax.text(0.5, 0.2, 'no clip')
t2 = ax.text(0.5, 0.3, 'clipped', clip_on=True)
```
Now pan the text to the data window borders and watch the clipping.

Possible resolution:
So the bug appears to be that the initial bounding box for text is not set if 'clip_on' is not supplied as an arg.

t1.get_clip_on() == True
t1.get_clip_box() == None
t2.get_clip_on() == True
t2.get_clip_box() == TransformedBbox(...)

Toggling clip_on does not fix the problem.