message: [boxdrawingrenderer] unriemannize this here manifold
diff: @@ -448,8 +448,8 @@ constexpr float operator/(int a, Ratio1 b) noexcept { return static_cast<float>(
  struct Ratio { float x; float y; };
  struct RatioBlock { Ratio from{}; Ratio to{}; };
  -constexpr RatioBlock lower(float r) noexcept { return RatioBlock{{0, 0}, {1, r}}; }
  -constexpr RatioBlock upper(float r) noexcept { return RatioBlock{{0, 1 - r}, {1, 1}}; }
  +constexpr RatioBlock lower(float r) noexcept { return RatioBlock{{0, 1 - r}, {1, 1}}; }
  +constexpr RatioBlock upper(float r) noexcept { return RatioBlock{{0, 0}, {1, r}}; }
  constexpr RatioBlock left(float r) noexcept { return RatioBlock{{0, 0}, {r, 1}}; }
  constexpr RatioBlock right(float r) noexcept { return RatioBlock{{1.f - r, 0.f}, {1.f, 1.f}}; }
  @@ -947,7 +947,7 @@ inline RatioBlock operator*(RatioBlock a, RatioBlock b) noexcept
  // 1 <= n <= r*n
  constexpr inline RatioBlock horiz_nth(float r, int n) noexcept
  {
  - return RatioBlock{{0, 1 - r * float(n)}, {1, 1 - r * float(n - 1)}};
  + return RatioBlock{{0, r * float(n - 1)}, {1, r * float(n)}};
  }
  constexpr inline RatioBlock vert_nth(float r, int n) noexcept
  