Cairo backend ignores alpha in imshow. 

As indicated in #1188 there is a problem with rendering images with alpha != 1 in the cairo backend. 
The following code produces an image with alpha = 1 using the cairo backend
but the right alpha on the colorbar.

```python
import matplotlib
matplotlib.use('cairo')
import numpy as np
import matplotlib.pyplot as plt

z = np.arange(150)
z.shape = (10,15)

fig, axs = plt.subplots(1,1)

ax = axs
im = ax.imshow(z, interpolation='nearest', alpha=0.3)
cbar = fig.colorbar(im, ax=ax)

plt.savefig("test2.png")
plt.savefig("test2.pdf")
plt.savefig("test2.svg")
plt.show()
```