widget: Add assertions to public API
widget: Add assertions to public API

Move the assertions to the public API instead of having them in the private
functions.

flaxandteal/gasket-vte,saitoha/vte-sixel,rvu95/vte,thestinger/vte-ng,flaxandteal/gasket-vte,rvu95/vte,rvu95/vte,rvu95/vte,jessevdk/vte,jessevdk/vte,thestinger/vte-ng,jessevdk/vte,flaxandteal/gasket-vte,flaxandteal/gasket-vte,saitoha/vte-sixel,jessevdk/vte,thestinger/vte-ng,saitoha/vte-sixel