Bug 1839459 - Elide empty closed sub-paths instead of emitting a start point. r=jrmuizel, a=dmeehan

There is a corner case that triggers an assert in wpf-gpu-raster if an empty sub-path
starts off a path. Just avoid the problem by eliding the sub-path, but at least handle
close correctly as per the Canvas2D closePath spec:

"The closePath() method, when invoked, must do nothing if the object's path has no subpaths.
Otherwise, it must mark the last subpath as closed, create a new subpath whose first point
is the same as the previous subpath's first point, and finally add this new subpath to the path."

Differential Revision: https://phabricator.services.mozilla.com/D181543