Fix ClientToScreen()/ScreenToClient() when used immediately after window creation.
And whenever window does not have an up-to-date GTK size allocation.
Closes #16061


git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_3_0_BRANCH@78033 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775