Write delay between frames correctly when saving GIF files.

Deal with delays greater than ~2.5s correctly, their most significant byte was
previously lost resulting in 0 delay being written to the file.

Closes #16392.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@76952 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775