“Avatar” continues to raise the bar on the home entertainment front.In less than three weeks, 20th Century Fox’s sci-fier has become the bestselling title ever in a comparable period, with 19.7 million copies sold worldwide. Sales include DVD and Blu-ray formats, with the latter accounting for 6.2 million units — more than double the usual share for high-definition discs. As a general rule, high-def copies represent about 7%-12% of all units sold. The record-setting pace for “Avatar” should come as no surprise. Since hitting DVD shelves on April 22, the film has been shattering records, selling 6.7 million copies in its first four days in North America. Blu-ray made up 2.7 million of that total, outpacing Warner Bros.’ “The Dark Knight,” which took 18 months to sell 2.5 million units. “Avatar” is selling faster than “Dark Knight,” which sold 16 million in its first three weeks. Overseas, “Avatar” displaced Warner’s “Harry Potter and the Order of the Phoenix” as the bestselling title since it launched on DVD. Given the speed at which “Avatar” has achieved record-breaking sales, coupled with additional homevid editions still to come, the James Cameron pic could potentially outrank Disney’s “Finding Nemo” as the top DVD title ever. “Nemo” has sold approximately 40 million copies since it hit DVD shelves in November 2003.