[geometry] Removed (need for) arrangment and moved disjoint.

Disjoint is now based on ratios too. All is now based on ratios, greatly
simplifying the original code with many lines and conditions.

test/strategies/segment_intersection_collinear.cpp

@@ -290,8 +290,9 @@ void test_all()
         'e', true, 0, 0,
         2, 0, 6, 0);
 
-    //   a1---------->a2
-    //   b2<----------b1
+    // Disjoint (in vertical direction, picture still horizontal)
+    //   a2<---a1
+    //                      b2<---b1
     test_segment_intersection<P>("case_recursive_boxes_1",
         10, 7, 10, 6,
         10, 10, 10, 9,