By GLENN KENNY  At one point in this film about protean musician Billy Preston, drummer and producer Bill Maxwell says, “Billy didn’t just have perfect pitch, he had perfect anticipation.” Meaning Preston could play a tune through with the other musicians after hearing only a few chords. And “Billy Preston: That’s the Way God Planned It,” a sometimes mind-blowing documentary directed by Paris Barclay, doesn’t ask you to take Maxwell’s word for it. The movie has abundant archival footage of...