Document and test behaviour of wxRegion methods when it is invalid.

Document which wxRegion methods can and can't be used when the region itself
is invalid.

Apply the minor changes to wxGTK (Xor() didn't do the right thing, Offset()
didn't assert) and wxOSX (Offset() crashed) to make their behaviour consistent
with wxMSW.

Add a (trivial, so far, but to be extended later) wxRegion unit test checking
that the methods do indeed behave as documented.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@69461 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775