Fixes a bug where the old list ring remained when a new formspec
was displayed over the old one. This created the list-ring of the new formspec
to be partly ignored.
Thanks to @VanessaE to report the bug, and @DonBatman to produce the code that
exposed it.
m_fields.clear();
m_boxes.clear();
m_tooltips.clear();
+ m_inventory_rings.clear();
// Set default values (fits old formspec values)
m_bgcolor = video::SColor(140,0,0,0);