There’s a heartbreaking documentary in the works on the equally heartbreaking game That Dragon, Cancer. The film is called Thank You For Playing and judging from the newly-released teaser trailer, it looks like it’ll be powerful stuff. Two independent filmmakers have captured the personal tragedy of Ryan Green, who is developing a nonfictional game based on his family’s struggles to cope with their infant son being stricken with terminal cancer. Green admits in the video that it was a tough call to allow strangers to haul in film equipment and point cameras at his son as he lay on a…