* First official portrait of Duchess of Cambridge * Subject said to be "very pleased" with outcome * Artist Paul Emsley said duchess a "generous" sitter * Public, critical reaction overwhelmingly negative By Mike Collett-White LONDON, Jan 11 (Reuters) - The first official portrait of Britain's Duchess of Cambridge, popularly known by her former name Kate Middleton, was unveiled in London on Friday, and opinion was sharply divided over an image many deemed unflattering.