“Onslaught,” Adam Wingard’s return to indie horror, sees the filmmaker reveling in newfound freedom after directing two “Godzilla x Kong” movies. Dripping with style, references and lots of gore, this John Carpenter-inspired kill fest puts its R-rating to the test. But it’s both too much, and not enough. Like its big baddies, the bruising “Onslaught” is lacking one major element: a brain.It has everything else though: German-language covers of “Paint it Black,” Drew Starkey with bleached eyebrows, a drug-fueled BDSM orgy, Adria Arjona in Snake Plissken drag, bit parts for beloved ‘80s action stars, a cute little kid, more slo-mo...