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

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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

Start by running the issue's orthographic Basemap example, first without imagery and then with m.shadedrelief(); compare the rendered extent with the llcrnrx, llcrnry, urcrnrx, and urcrnry bounds. Check the shadedrelief and bluemarble entry points and verify that the background is clipped to the requested map region while preserving the existing projection behavior.

Written by the indexing model from the issue text.