Bug with PatchCollection in PDF output

The following script:

```
import numpy as np
np.random.seed(12345)

from matplotlib.patches import Circle
from matplotlib.collections import PatchCollection
import matplotlib.pyplot as plt

c = Circle((8, 8), radius=4, facecolor='none', edgecolor='green')
print(c.get_fill())
p = PatchCollection([c], match_original=True)

fig = plt.figure()
ax = fig.add_subplot(1,1,1)
ax.imshow(np.random.random((16,16)), interpolation='nearest')
ax.patch.set_color('0.5')
ax.add_collection(p)
fig.savefig('test.pdf')
```

Produces the following figure:

![test](https://f.cloud.github.com/assets/314716/307869/ba5371f2-96d1-11e2-8510-4b30725af5d5.png)

so it looks like there is something weird going on - the circle is cutting a hole through the image! This is with the latest dev version, but I think the bug affects 1.2.1 too.