baselayers incorrect for some 'sper' projections.

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.

Reproduce the issue with the two Basemap examples, comparing shadedrelief, bluemarble, and etopo when nsper uses explicit llcrnrx, llcrnry, urcrnrx, and urcrnry values. Start from the Basemap projection and shadedrelief entry points, using the linked gist and screenshot as references. Done means the base layers display correctly with the explicit corner coordinates.

Written by the indexing model from the issue text.