Make sure that canvasx and canvasy properties are initialized from the start (#896)

Make sure that canvasx and canvasy properties are initialized from the start (#896)

Adding fields to a structure after it is created can be slow on some
browsers, like Chrome.

Adding a demo to benchmark many points, this seems to speed up my
Chromium from about 2.8-3s to about 1.8s.

Thanks to Christopher Palmer (@thecav) for the analysis and specific
suggestion.