cgifsave: theshold alpha and check alpha constraint in one pass (#2827)
cgifsave: theshold alpha and check alpha constraint in one pass (#2827)

* cgifsave: theshold alpha and check alpha constraint in one pass

* cgifsave: get rid of sqrt()
lovell/libvips,lovell/libvips,lovell/libvips,lovell/libvips