Fix positioning of radio buttons inside wxOSX radio boxes in some cases.

If the previous radio box size was too small (notably (0,0) as it happened
during AUI relayout), the buttons were positioned without taking the box
margins into account, resulting in visually broken display.

Recompute the full size after setting the new size but before positioning the
buttons in wxRadioBox::DoSetSize() to avoid this.

Closes #14087.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_3_0_BRANCH@75149 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775