debugutils: improve dot file flow layout
debugutils: improve dot file flow layout

Iterate source- and sink-pads separately to ensure that the graph reflects the
upstream/downstream order. Fixes #643269

StreamUtils/gstreamer,lubosz/gstreamer,cablelabs/gstreamer,shelsonjava/gstreamer,drothlis/gstreamer,surround-io/gstreamer,ylatuya/gstreamer,Distrotech/gstreamer,centricular/gstreamer,Distrotech/gstreamer,lovebug356/gstreamer,magcius/gstreamer,krieger-od/gstreamer,drothlis/gstreamer,StreamUtils/gstreamer,magcius/gstreamer,lovebug356/gstreamer,drothlis/gstreamer,justinjoy/gstreamer,cfoch/gstreamer,ylatuya/gstreamer,cablelabs/gstreamer,collects/gstreamer,mparis/gstreamer,cablelabs/gstreamer,magcius/gstreamer,centricular/gstreamer,collects/gstreamer,mparis/gstreamer,surround-io/gstreamer,krieger-od/gstreamer,drothlis/gstreamer,justinjoy/gstreamer,Lachann/gstreamer,Lachann/gstreamer,Distrotech/gstreamer,shelsonjava/gstreamer,krieger-od/gstreamer,ensonic/gstreamer,jpxiong/gstreamer,centricular/gstreamer,cfoch/gstreamer,krichter722/gstreamer,Lachann/gstreamer,shelsonjava/gstreamer,lubosz/gstreamer,lubosz/gstreamer,mparis/gstreamer,ensonic/gstreamer,ylatuya/gstreamer,justinjoy/gstreamer,jpakkane/gstreamer,Distrotech/gstreamer,lubosz/gstreamer,krichter722/gstreamer,lovebug356/gstreamer,Lachann/gstreamer,ensonic/gstreamer,jpxiong/gstreamer,cablelabs/gstreamer,ensonic/gstreamer,StreamUtils/gstreamer,surround-io/gstreamer,magcius/gstreamer,jpakkane/gstreamer,jpxiong/gstreamer,collects/gstreamer,magcius/gstreamer,centricular/gstreamer,ylatuya/gstreamer,krieger-od/gstreamer,lubosz/gstreamer,surround-io/gstreamer,drothlis/gstreamer,shelsonjava/gstreamer,krichter722/gstreamer,jpakkane/gstreamer,collects/gstreamer,krichter722/gstreamer,ensonic/gstreamer,StreamUtils/gstreamer,cfoch/gstreamer,jpakkane/gstreamer,lovebug356/gstreamer,mparis/gstreamer,krieger-od/gstreamer,jpakkane/gstreamer,ylatuya/gstreamer,cfoch/gstreamer,collects/gstreamer,Distrotech/gstreamer,shelsonjava/gstreamer,justinjoy/gstreamer,lovebug356/gstreamer,centricular/gstreamer,Lachann/gstreamer,mparis/gstreamer,surround-io/gstreamer,justinjoy/gstreamer,cfoch/gstreamer,cablelabs/gstreamer,StreamUtils/gstreamer,krichter722/gstreamer,jpxiong/gstreamer,jpxiong/gstreamer