LITTLETON, N.H. — Few were present at the select board meeting in Littleton, New Hampshire, in August when Carrie Gendreau, one of its members, began to talk about a mural that had recently been painted on the side of a building downtown. Until that moment, it had not attracted much attention. Its subject matter — a blooming iris, dandelions, birch trees — did not seem controversial. But for Gendreau, 62, who was also a state senator representing northern New Hampshire, the mural had set off ala