outlined-equilateral-triangle

;;; (outlined-equilateral-triangle width height color line-width [desc]) -> image?
;;;   width : nonnegative-real?
;;;   height : nonnegative-real?
;;;   color : color?
;;;   line-width : positive-integer?
;;;   description : string?
;;; An outlined equilateral-triangle whose inner size is width-by-height with
;;; an outline of `line-width`. The inner triangle is transparent.