markerfacecolor="None" becomes black when using the alpha channel #11458

Bug summary

When plotting non-filled markers with markerfacecolor="None" along with alpha=non-zero the marker face color becomes black for the png format. Same happens when using plt.show().

Code for reproduction

import matplotlib.pyplot as plt

plt.errorbar([1], [1], markerfacecolor = "None",
        marker = 'o', alpha = 0.9, markersize=100)

plt.savefig('test.png')
plt.savefig('test.pdf')

Actual outcome

Expected outcome
test.pdf

Matplotlib version

matplotlib 2.2.2-2 on arch-linux