Fix X 'BadPixmap' crash in Blit() with mask when GTK+ <= 2.16, closes #16303


git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_3_0_BRANCH@76626 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775