slideshow: another attempt to fix letter box clipping

The workaround in 16552e9a67 wasn't right. This commit fixes
the bug, which was that the clipping region wasn't reset correctly,
instead f trying to work around it.
macro expander: restore a minor shortcut lost in a previous commit

The commit c352ef8f lost a shortcut that is used for looking up
certain bindings in an environment during expansion/compilation.
fix an inconsistency in `free-identifier=?`

Closes PR 13982