pcolormesh does not work with south pole ortho projection

Personne n'a encore pris cette issue.

The north pole works fine when setting lat_0=90 but when setting lat_0=-90 it doesn't work. The data shown in the southern projection is nonsense.

Related to #347 and #350 but the workaround described there for the "stereo" projection does not work for the "ortho" projection.

Code to replicate:

```
from mpl_toolkits.basemap import Basemap
import numpy as np
import matplotlib.pyplot as plt

lons = np.arange(-179.95, 180.01, 0.1)
lats = np.arange(-89.95, 90.01, 0.1)

# m = Basemap(projection='ortho', lon_0=-270, lat_0=-90., resolution='c')
m = Basemap(projection='ortho', lon_0=0, lat_0=90., resolution='c')
data = np.random.random((1800, 3600))
lons, lats = np.meshgrid(lons, lats)
x, y = m(lons, lats)

# data = np.ma.masked_where((x < m.xmin) | (x > m.xmax) | (y < m.ymin) | (y > m.ymax), data)

im = m.pcolormesh(x, y, data, latlon=False, cmap='RdBu')

m.colorbar(im)
m.drawcoastlines()
m.drawcountries()
plt.show()
```

Commencez par la reproduction fournie de la projection ortho de Basemap, en comparant lat_0=90 à lat_0=-90 via le chemin pcolormesh. Examinez les issues associées #347 et #350, puis vérifiez que la projection sud affiche correctement les données sans perturber le comportement existant de la projection nord.

Rédigé par le modèle d'indexation à partir du texte de l'issue.