Use client size determined by size-allocate when possible

Use client size determined by size-allocate when possible

Our client size calculation can be wrong for reasons beyond our control,
notably on Ubuntu 12.04 with overlay scrollbars where the GtkScrolledWindow
"scrollbar-spacing" is wrong. By using the true size from size-allocate
when it becomes available, we can recover from an earlier incorrect client
size calculation. See #14871

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@73231 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775