Strange lines appear when using pcolormesh with nonzero alpha value

还没有人认领这个 Issue。

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?

首先运行提供的、使用 Robinson 投影的 Basemap.pcolormesh 示例，比较 alpha 小于 1 与 alpha=1 的情况。跟踪 pcolormesh 调用进入 matplotlib 渲染路径，并找出为什么投影后的单元格在透明度合成期间会产生可见线条。使用非不透明 alpha 时，复现出的图中没有非预期线条，即表示完成。

由索引模型根据 Issue 内容生成。