Rather than use an inline wrapper and rely on the compiler optimizer to
redirect gr_set_current_canvas(nullptr) to gr_set_default_canvas,
rewrite all relevant calls directly in the source.
git grep -l 'gr_set_current_canvas' | xargs sed -i -e 's:gr_set_current_canvas(\s*NULL\|nullptr\s*);:gr_set_default_canvas();:'