A dour, grey-and-blue-hued high school. A cold house. The vibrantly red backroom of a church. The pulsating, glittery universe of a drag ball. These are the worlds a black, gay teenager claims in Damon Cardasis’ spirited “Saturday Church.” The film attempts to cohere myriad emotional complexities in just 80 minutes, and it half-succeeds. Like many first films, “Saturday Church” occasionally gives way to melodrama or loses track of its onerous plot.