Errorbar markers not drawn in png output

Using the current master (0a96efb), no markers are drawn when calling `errorbar()` with either `mec='None'` or `mew=0` and outputting to png.  For some reason, pdf output seems unaffected.

The following code reproduces the error:

``` python
import matplotlib.pyplot as plt
import numpy as np

plt.rcdefaults()

x = np.arange(10)
y = x.copy()
dy = y*0.1

plt.errorbar(x, y, yerr=dy, marker='s', mew=0)
plt.errorbar(x, y+1, yerr=dy, marker='o', mec='None')

plt.savefig("test_markers.png")
plt.savefig("test_markers.pdf")
```