Fix Firefox canvas draw bug.
Fix Firefox canvas draw bug.

macGRID-SRN/streetmix,macGRID-SRN/streetmix,magul/streetmix,CodeForBrazil/streetmix,codeforamerica/streetmix,CodeForBrazil/streetmix,macGRID-SRN/streetmix,magul/streetmix,codeforamerica/streetmix,kodujdlapolski/streetmix,codeforamerica/streetmix,magul/streetmix,kodujdlapolski/streetmix,kodujdlapolski/streetmix,CodeForBrazil/streetmix