Don't change client size of TLW when centering it in wxGTK.

Don't change client size of TLW when centering it in wxGTK.

Setting the frame client size and then calling Centre() on it resulted in the
frame coming up with a wrong client size in wxGTK because the call to Centre()
resulted in calling wxTopLevelWindow::DoSetSize() which unconditionally reset
m_deferShowAllowed to true, undoing setting it to false in DoSetClientSize().

Only reset m_deferShowAllowed if the size was really changed but not if the
window was only moved, as happens when centering it.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@72170 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775