For something impressively unguarded about “Filth.” Sophomore helmer Jon S. Baird’s needfully messy adaptation of “Trainspotting” author Irvine Welsh’s 1998 bestseller stays true to the novel’s spirit of riotous despair, even as it cuts down on the tapeworm monologues. Powered by a vigorous, image-shedding lead turn from James McAvoy as a coked-up Edinburgh detective on the fast track to either promotion or self-implosion, this descent into Scotch-marinated madness begins as ugly comedy, segues almost imperceptibly into farcical tragedy, and inevitably — perhaps intentionally — loses control in the process. Released across Blighty in early October, this bracingly abrasive trip has shown surprising commercial legs for a film that, true to its title, leaves audiences craving a hot shower afterward; Yanks may be less inclined to savor the stink.