Correct test for parent being on screen in wxTLW::DoCentre().

Correct test for parent being on screen in wxTLW::DoCentre().

The old test was needlessly complicated, didn't take all cases into account
and had a typo in it which prevented it from working in half of the cases
which it did [try to] take into account.

Correct this by simply checking if the parent rectangle intersects the visible
display area at all.

Also simplify the code a little by checking for the window being maximized
from the beginning as centering a maximized window on either screen or parent
is a no-op.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@63171 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775