emulation: Improve DECSET handler
emulation: Improve DECSET handler

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pjm0616/vte,pjm0616/vte,rvu95/vte,jessevdk/vte,rvu95/vte,saitoha/vte-sixel,rvu95/vte,pjm0616/vte,jessevdk/vte,saitoha/vte-sixel,flaxandteal/gasket-vte,flaxandteal/gasket-vte,rvu95/vte,flaxandteal/gasket-vte,jessevdk/vte,jessevdk/vte,thestinger/vte-ng,thestinger/vte-ng,flaxandteal/gasket-vte,thestinger/vte-ng,saitoha/vte-sixel