Improve NavigationObstacle2D debug visuals and performance

[smix8]

Improves NavigationObstacle2D debug visuals by aligning them closer to their aleady improved 3D counterpart.

Also improves performance on larger obstacles due to using a simple static line mesh instead of inefficient canvas item polygon and polyline drawing. It now shows the edge winding with a small "arrow" aka if the obstacle pushes out or in. It also colors in red when the vertices are invalid, aka fail the triangulation due to e.g. crossed edges.

To improve performance for both the 2D and 3D obstacle also adds internal functions to avoid recalculating the vertices winding and validity all the time. They are now calculated once vertices are set with the result cached and available to query with two new internal functions on both the 2D and 3D obstacle.

bool are_vertices_clockwise() const { return vertices_are_clockwise; };
bool are_vertices_valid() const { return vertices_are_valid; };

[Repiteo]

Thanks!