Replace GDK_THREADS_ENTER/LEAVE with gdk_threads_enter/leave
Replace GDK_THREADS_ENTER/LEAVE with gdk_threads_enter/leave

Using the functions directly allows us to benefit from the
new more gracious deprecation approach, and doesn't force
a harsh build break due to undefined macros.

https://bugzilla.gnome.org/show_bug.cgi?id=681216

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