When a CollisionPolygon3D is drawn with a concave area, it fails to render the surface of the top and bottom.
Steps to reproduce the bug in the editor:
1. Create a new scene
2. Create CollisionPolygon3D
3. Make sure debug fill is checked in the CollisionPolygon3D inspector.
4. Add these vertices to the the polygon (0,0), (1,-1), (1,1), (-1, 1), (-1, 0.3).
You should see that only one triangle from the top and bottom of the polygon is drawn and the reset are missing.
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