baselayers incorrect for some 'sper' projections.

还没有人认领这个 Issue。

This may be related to #168, but might not.

If your 'sper' basemap is set up to choose corners using xy values, then the baselayers
are not displayed properly. These all fail: bluemarble, shadedrelief, and etopo.

Any help would be great.
Thanks

Does what I expect:

``` python
h = 15000.; lon_0 = -115; lat_0 = 55

m = Basemap(projection='nsper',satellite_height=h*1000.,\
lon_0=lon_0,lat_0=lat_0,resolution='l')
m.drawparallels(np.arange(-90.,120.,30.))
m.drawcoastlines()
m.shadedrelief(scale=.2)
```

Fails to correctly display the base layer.

``` python
m = Basemap(projection='nsper',satellite_height=h*1000.,\
lon_0=lon_0,lat_0=lat_0,resolution='l',\
llcrnrx=-4000000,llcrnry=0,urcrnrx=4000000,urcrnry=5000000)
m.drawparallels(np.arange(-90.,120.,30.))
m.drawcoastlines()
m.shadedrelief(scale=.2)
```

Here's a gist with what I'm seeing
https://gist.github.com/flamingbear/76eeb588c006965361c6

and a screenshot.

使用两个 Basemap 示例重现该问题，在 nsper 使用 llcrnrx、llcrnry、urcrnrx 和 urcrnry 的显式值时，对比 shadedrelief、bluemarble 和 etopo。从 Basemap 投影和 shadedrelief 的入口点开始，并以链接的 gist 和屏幕截图作为参考。当基础图层使用显式角点坐标正确显示时，即表示完成。

由索引模型根据 Issue 内容生成。