Food glorious food . . . that will be the message at the Convention Centre next week when Food Expo '94 opens. And for some, like Leong Ngai-fong, it will be a chance to display their culinary art. Leong, pictured above with a baked offering he calls ''Bread as Seafood'', promises more delights from his team of bakers at Taipan Bread and Cakes. Using 955 kilograms of dough, they are working on a 6.1-metre tall, 24.4-metre long ''Bread Wall Of China'' that will be a highlight of the exhibition, which opens next Wednesday. It remains to be seen if the wall will succeed in keeping the invading hungry hordes to Food Expo '94 at bay.