Misc. spline extrusion properties and expose spline index

[GuitarBro]

Just a few changes I found handy when using SplineMesh in a personal project. Figured they might be useful for other people too.

• Added extrusion layer/visibility/collision properties since I wanted some collision-only and visible-with-no-collision splines.
• Exposed spline index so my game logic could know if one spline came before/after another one.