BANGKOK — How Come Entertainment Co., partly owned by Thai Prime Minister Thaksin Shinawatra’s son, is forming a joint venture with Hong Kong’s Media Asia, producer of the “Infernal Affairs” franchise.As yet unnamed, the new entity will have an initial capitalization of $10 million. It aims to produce five to six films a year, initially in Thai. The partners say they are also looking at the possibility of shooting pics in English and Chinese. How Come will own half of the co-venture and oversee distribution in Asia, while Media Asia will handle sales in the rest of the world. The 25-year old Panthongtae Shinawatra, the PM’s only son, launched How Come a year ago, and owns a 20% stake. His sister Pinthongta Shinawatra has 16%.