use float div for sigma calc in gaussblur
use float div for sigma calc in gaussblur

thanks aferrero2707

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