Follow-up to #3697 (CodeRabbit): a brush stroke is drawn as separate
pieces (draw_brush), so folding object opacity into its colour isn't exact.
A non-dashed brush stroke already counted as one paint before #3697; to
keep that change's scope, a dashed stroke counts as one paint only when it
has no brush. dashed_stroke_opacity_folds_into_its_colour checks that a
dashed brush stroke isn't folded; it fails without the change.