There may be more important works of art, like the David or the Last Supper or the Sistine Chapel, but for sheer charm there's nothing like these 15th-century frescoes by Mantegna. You find them not in a mobbed museum but in a quiet jewel box, a small chamber in the ducal palace of Mantua. The brightly colored paintings literally shimmer with inlaid gold and lapis. The paintings depict nobility, courtiers, children, dwarves, horses, dogs--all watched over not by a stern God but by an assortment