pdf backend not outputting masks for grayscale images

[jkseppan]

Reported in http://thread.gmane.org/gmane.comp.python.matplotlib.general/35473/

@tacaswell provided this example where you get artifacts in the pdf output:

x, y = np.ogrid[-5:5:.1, -5:5:.1]
dd = np.exp(-(x**2 + y**2))
dd[dd < .1] = np.nan

fig, ax = plt.subplots()
ax.imshow(dd, interpolation='none', cmap='gray_r')
plt.savefig('test.pdf')

If you change the colormap to e.g. 'Greens' the artifacts go away. I think this is because of the asymmetry in PdfFile.writeImages: grayscale images don't get an smask object while rgb images do.