Painted crossing points (nodes) with filled asphalt color.
Corrected direction of crossing test for edges by reversing.
Scaled up the whole simulation and avoided too near nodes to be created.
Made Graph::update() to retry creating valid Vehicles even if failed.

Now we can get really crowded simulation even if the graph scale is
large.  Be aware that there could be a condition of parameters that induce
infinite or very long loop for never giving up retry logic.
Added vec2.h, a class template for 2-D vector into cpplib.
Maybe it's too late, but I'm adding .gitattributes to normalize eol.
Made use of Vec2d to some extent.