The Canvas2D specification says that if a path has no active sub-paths, and a
primitive is drawn, that the first point of that primitive becomes the start of
the newly created sub-path that will be created for it.
So if we prune a point when a path has no active sub-paths, and then a new
primitive comes in that does not start with that same point, we risk not
installing the pruned point as the start of that new sub-path.
To solve this, we need to detect if a path has no active sub-paths while
we are building it. This adds PathBuilder::IsActive() to help with that.
Then before we go to add a primitive, we check if there is a pruned point
on a path that is not active yet, and if so, install the correct start
point with a MoveTo.
This also makes IsActive and IsEmpty required so to ensure all our path
implementations behave consistently rather than having any surprising
unimplemented behavior.
Differential Revision: https://phabricator.services.mozilla.com/D184891
b2d67698b9