robust 'flip y direction' for drawing to mplfig

Commit ee3b35bc fixed plotting on matplotlib figures when the extent was fitted to the vertex positions. I think the following is a more general solution:

-    y.fa *= -1
-    y.fa -= y.fa.min()
+    l, t, r, b = ctx.clip_extents()
+    y.fa = b + t - y.fa

This even worked for me when mplfig was a cartopy.mpl.geoaxes.GeoAxes instance.