Fix differenced redraw artifacts during pan/zoom
Fix differenced redraw artifacts during pan/zoom

Fixes #543.

ngageoint/hootenanny-ui,openstreetmap/iD,zbycz/iD,AndreasHae/iD,kartta-labs/iD,morray/iD,ngageoint/hootenanny-ui,1ec5/iD,mapmeld/iD,openstreetmap/iD,digidem/iD,1ec5/iD,ngageoint/hootenanny-ui,AndreasHae/iD,morray/iD,bagage/iD,edpop/iD,zbycz/iD,andreic-telenav/iD,mapmeld/iD,AndreasHae/iD,energiekollektiv/openmod.sh-id,andreic-telenav/iD,edpop/iD,openstreetmap/iD,1ec5/iD,kartta-labs/iD,bagage/iD,mapmeld/iD,edpop/iD,tbicr/iD,energiekollektiv/openmod.sh-id,tbicr/iD,bagage/iD,kartta-labs/iD,kepta/iD,digidem/iD,kepta/iD