add tf_x_twist

src/experimental/tf_x_twist.scad

@@ -0,0 +1,16 @@
+use <rotate_p.scad>;
+
+/*
+    size: The size of a rectangle.
+    point: A point in the rectangle.
+    angle: twisted angle.
+*/
+function tf_x_twist(size, point, angle) =
+    let(
+        xlen = size[0],
+        ylen = size[1],
+        y_offset = ylen / 2,
+        a_step = angle / xlen,
+        y_centered = [point[0], point[1], 0] + [0, -y_offset, 0]
+    )
+    rotate_p(y_centered, [point[0] * a_step, 0, 0]) + [0, y_offset, 0];