Fix NavigationRegion3D gizmo's odd visual behavior

[DarkKilauea]

Does the following:

• Use gizmo managed materials so the gizmo's xray setting and selected status apply correctly to the visualization for the navigation mesh.
• Use the same technique for computing random face colors as NavigationRegion2D to avoid cases where a polygon appears to be missing due to it being mostly transparent.
• Slightly increase the variability of the face colors for both the 2D and 3D versions of NavigationRegion.