Use standard fillRule property if available, fallback to mozFillRule

*update*: Change proposed property name to fillRule (was webkitFillRule) since this ought to be a standard attribute.

For some PDF files the mozFillRule property of the HTML5 canvas is used which is a non-standard property only available in Firefox and other Mozilla-based browsers.  This patch allows the rendering of such PDF files to use the new standard fillRule property if that property is available and only falls back to mozFillRule if that isn't available.

Note that, at the time of writing, no known browser implements the fillRule attribute.  I am preparing this pull request in anticipation that maybe sometime soon one or two will.

See also https://github.com/mozilla/pdf.js/issues/2351