Ortho projection fails at south pole in pcolormesh

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I am plotting gridded satellite data using basemap 1.0.7 (I can't get 1.0.8 via pip yet). The grid is 0.25 degrees resolution, global coverage.

For polar views, I use pcolormesh. It works fine for the north polar view:

```
bmap = Basemap(projection='ortho', lat_0=90, lon_0=270, resolution='c')
show_map = bmap.pcolormesh(lons, lats, darray, vmin=vminval, vmax=vmaxval,
norm=norm, cmap=this_cmap, alpha=1.0, latlon=True)
```

but the corresponding map centered on the south pole plots garbage:

```
bmap = Basemap(projection='ortho', lat_0=-90, lon_0=270, resolution='c')
show_map = bmap.pcolormesh(lons, lats, darray, vmin=vminval, vmax=vmaxval,
norm=norm, cmap=this_cmap, alpha=1.0, latlon=True)
```

However, pcolor plots correctly at the south pole. This one is fine:

```
bmap = Basemap(projection='ortho', lat_0=-90, lon_0=270, resolution='c')
show_map = bmap.pcolor(lons, lats, darray, vmin=vminval, vmax=vmaxval,
norm=norm, cmap=this_cmap, alpha=1.0, latlon=True)
```

In this case, my gridded data array is pretty small and the time difference between pcolormesh and pcolor is negligible, but I'm guessing this is something that ought to be fixed, unless it already has been addressed in 1.0.8.

Riproduci il problema con gli esempi Basemap forniti per il polo nord e il polo sud usando pcolormesh e pcolor, quindi esamina l’implementazione di pcolormesh di Basemap e la gestione della proiezione. La correzione è completa quando pcolormesh ortografico del polo sud produce la stessa mappa corretta di pcolor senza introdurre regressioni nel caso del polo nord.

Scritto dal modello di indicizzazione a partire dal testo della issue.