Add macro cf_assert ("control flow" assert) to hint to gcc that certain
conditions are impossible. Use it to avoid generating range checks for
situations that never happen. If the event did happen, the only
consequence would be truncated UI text, rather than a correctness
problem.
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();:'