Strange lines appear when using pcolormesh with nonzero alpha value

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When plotting data using pcolormesh on a basemap projection (or a cartopy projection) I notice strange lines appear when I set the alpha value to less than 1.

Example code:

```
from mpl_toolkits.basemap import Basemap
import numpy as np
import matplotlib.pyplot as plt

plt.clf()

dpp =1 # degrees per pixel
lons = np.arange(-180,180+dpp,dpp)
lats = -1*np.arange(-90,90+dpp,dpp)

m = Basemap(projection='robin',lon_0=0)
data = np.random.random((np.size(lats), np.size(lons)))
lons, lats = np.meshgrid(lons, lats)
x, y = m(lons, lats)

im = m.pcolormesh(x, y, x, latlon=False, cmap='RdBu')
#im = m.pcolormesh(lons, lats, data, latlon=True, cmap='RdBu')

m.colorbar(im)
plt.show()
```

The output shows strange lines appearing:


If I instead set alpha=1 the lines disappear and the behavior is as expected:


Any ideas on what the issue is or how to get pcolormesh to work with a nonzero alpha value?

Start by running the supplied Basemap.pcolormesh example with the Robinson projection, comparing alpha below 1 with alpha=1. Trace the pcolormesh call into the matplotlib rendering path and identify why projected cells produce visible lines during transparency compositing. Done means the reproduced plot has no unintended lines when using nonopaque alpha.

Written by the indexing model from the issue text.