Problem using Ortho projection and shadedrelief on a sector of the globe

还没有人认领这个 Issue。

I'm trying to plot a sector of the world in basemap using an orthographic projection and I would like to have a shadedrelief background.

If I don't use shadedrelief everything works fine:
```
import numpy as np
from mpl_toolkits.basemap import Basemap
import matplotlib.pyplot as plt

fig = plt.figure()
ax = fig.add_subplot(111)

m = Basemap(projection='ortho',lon_0 = 0, lat_0 = 40,
llcrnrx=-3000000, llcrnry=1000000, urcrnrx=3000000, urcrnry=7000000,
resolution = 'l')

m.drawcoastlines()
m.fillcontinents(color='coral',lake_color='aqua')

# draw parallels and meridians.
m.drawparallels(np.arange(-90.,120.,30.))
m.drawmeridians(np.arange(0.,420.,60.))
m.drawmapboundary(fill_color='aqua')
```
[![Orthographic projection without shadedrelief background][1]][1]

However, when I add the command `m.shadedrelief()` (after removing the `m.drawmapboundary` and the `m.fillcontinents` commands) I get the following graph:
[![Orthographic projection with shadedrelief background][2]][2]

Apparently the problem is that the background is not restricted to the bounding box I set in the basemap command. I tried also with other backgrounds (e.g. bluemarble) but I get the same problem.

p.s. This is a copy of a thread that I opened on [stackexchange](https://gis.stackexchange.com/questions/298680/setup-shadedrelief-background-in-basemap-using-a-cropped-orthographic-projection), sorry for the cross-posting!

[1]: https://i.stack.imgur.com/iON27.png
[2]: https://i.stack.imgur.com/oVMAI.png

首先运行 issue 的正射 Basemap 示例，先不使用影像，然后使用 m.shadedrelief()；将渲染范围与 llcrnrx、llcrnry、urcrnrx 和 urcrnry 边界进行比较。检查 shadedrelief 和 bluemarble 入口点，并验证背景是否被裁剪到请求的地图区域，同时保留现有的投影行为。

由索引模型根据 Issue 内容生成。