Self shadows should be disabled for convex primitives

[omichel]

Spheres, Cylinders, Boxes, Capsules, Cones, Planes, convex IndexedFaceSets and convex ElevationGrids should not cast their own shadows on themselves. This consumes CPU/GPU resources uselessly and cause some display bugs like this:

[omichel]

Unfortunately, this is a known limitation of stencil shadows and cannot be corrected easily.
This is well explained here and here (french). An idea for a possible solution is explained here, but needs to be investigated so ensure it's doable and doesn't have nasty side effects.