CPU 100% usage while idling on some backends.

I get 100% CPU usage just opening a single plot like the following, and 200% running from two separate python instances (dual-core):

``` python
import matplotlib.pyplot as plt
plt.figure()
plt.plot(range(5))
plt.show()
```

I have tested this on `GTK` and `GTK3`, both exhibit this, and on `QT4` which does not.
