Navigation2D get_simple_path() does not always find the shortest path

[timcode77]

Godot version:
3.3.stable.official

Issue description:
Navigation2D get_simple_path() does not always find the shortest path.

These may be related:
#15919
#17885

Steps to reproduce:
(see demo project / screengrabs)

1. Create a Navigation2D Node with a NavigationPolygonInstance as a child.
2. For the NavigationPolygonInstance, draw a shape with 4 vertices and a triangle inside it.
3. Call get_simple_path for two points inside the NavigationPolygonInstance.
4. The path generated might not be the shortest path.

See images from my test project that demonstrates the problem:

Shortest path:

Not shortest path:

Minimal reproduction project:

nav_problem.zip

[Calinou]

Duplicate of #19011, #28235 and possibly others.