Experimentally adjust camera up vector depending on kart roll, which means

the camera will now follow the gravity changes.

src/graphics/camera.cpp

@@ -534,6 +534,9 @@ void Camera::positionCamera(float dt, float above_kart, float cam_angle,
         }
     }
  
+    // Rotate the up vector (0,1,0) by the rotation ... which is just column 1
+    Vec3 up = m_kart->getTrans().getBasis().getColumn(1);
+    m_camera->setUpVector(up.toIrrVector());
 }   // positionCamera
 
 // ----------------------------------------------------------------------------