Oriented SceneObjectModel/SceneObjectPolygon measure depth from the world origin #342

Bug

In oriented draw mode, SceneObjectModel and SceneObjectPolygon measure their camera depth from the world origin instead of from the object.

Both override getPositionForCameraDistance() to return m.worldPoints.getCenter() when drawMode = oriented (SceneObjectModel.bs:330-335, SceneObjectPolygon.bs:88-93). worldPoints is inherited from SceneObjectBillboard, but only SceneObjectBillboard.updateWorldPosition() fills it. Both subclasses override updateWorldPosition() without calling super, and fill modelWorldFaces/modelWorldBounds or polygonWorldPoints instead. So worldPoints keeps its default zero vectors, and depthPosition/negDistanceFromCamera come out as the distance to (0, 0, 0).

Effect

• An oriented model or polygon depth-sorts as though it were at the world origin, so it can draw in front of or behind other objects incorrectly.
• The behind-the-camera skip (negDistanceFromCamera >= 0) and BSP dynamic-item classification (getDepthPosition()) use the same wrong point.
• Found while planning Investigate occlusion culling for the renderer #108: occlusion culling needs a correct depth for static models acting as occluders.

Fix

• Return a real centre for these classes in oriented mode: the centre of modelWorldBounds for models, and the centroid or bounds centre of polygonWorldPoints for polygons.
• Add specs asserting that an oriented model/polygon placed away from the origin reports a depth based on its own position.