For the Belton High School boys basketball team, the writing isn't on the wall.It's on the Tigers' backs.Namely, it's on the T-shirts they're sporting this season, the ones bearing the phrase "On a Mission: 35 Years in the Making."When 13th-ranked Belton (18-4) begins the District 17-4A season at 8 p.m. today at Killeen Shoemaker (11-10), the Tigers will be gunning for their first outright district title since the 1965-66 season. Belton has shared the title five times since then, the last time coming in 1990.The Tigers, who finished third in 17-4A last season and ended a nine-season playoff drought, have established themselves as the district front-runner with their impressive non-district record. Belton's four losses are to Class 5A teams