Unify duplicate Corsica & Sardinia island generation, refs rP11266.

The code for the other island was copied in a varying order with only the values changed to 1 - x.
Unite the island, subisland, creeks, beaches, main relief, first plateau, second plateau and passagess code under a single loop to display the logical relation
instead of scattering many paragraphs and executing unrelated (starting base) code in between.

Cleanup affected createArea calls as in rP20301.