plantuml 7994

Closes #27239.

Signed-off-by: Mike McQuaid <mike@mikemcquaid.com>
mlt: allow GTK enabled builds

Without GTK being enabled there are no loaders for PNG images, making
things less than useful.

Enable building --with-gtk

Closes #27240.

Signed-off-by: Mike McQuaid <mike@mikemcquaid.com>