refactor im_aconv
refactor im_aconv

get ready for rolling vertical sums too

lovell/libvips,wishabi/libvips,wishabi/libvips,mcanthony/libvips,leiyangyou/libvips,wishabi/libvips,jcupitt/libvips,jcupitt/libvips,lovell/libvips,jcupitt/libvips,lovell/libvips,leiyangyou/libvips,leiyangyou/libvips,mcanthony/libvips,mcanthony/libvips,jcupitt/libvips,leiyangyou/libvips,lovell/libvips,wishabi/libvips,mcanthony/libvips