Unbreak physics debugger with projectiles' spherical shapes.

Since !456 (closed), projectiles got a physical spherical shape. The debug drawer doesn't works with them:

1. the shape are not rendered correctly (or at all)
2. after a while it generates an error like

   CullVisitor::apply(Geode&) detected NaN,
        depth=nan, center=(nan nan nan),
        matrix={
           -0.265814 -0.0639702 0.9619 0
           0.964024 -0.0176387 0.265228 0
           -4.29769e-09 0.997796 0.0663574 0
           18178.6 -3550.91 42154.4 1
    }

The debugger will be non-functionnal until engine restart (at least on my machine, nan handling is configuration-dependant so YMMV).

Use osg::Sphere instead of Bullet-supplied vertices to solves this.