Double Chrome hextile perf again. Add canvas test.

Double Chrome hextile perf again. Add canvas test.

- By dereferencing the 'data' field of the imageData object before the
  loop, the hextile performance on Chrome is down to 140ms or so for
  a full 800x600 update. Still have to fall back to Canvas operations
  for firefox.

- Fix RQ empty after reorder bug.