Fix ribbon bar tab width computation if the first tab is hidden.

The logic for the first tab should really deal with the first visible tab as
the actual first tab might be hidden and in this case we shouldn't account for
it at all.

Closes #16432.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_3_0_BRANCH@77092 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775