Let the image list deduce its size from the first bitmap in it. This is better
than the old behaviour of using the standard icon size as it allows to omit
the size from the image lists provided they contain the bitmaps of the same
size.
git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@65884 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775