Bug 1546894 - Keep track of current and begin points in path objects in case we'll create a builder from them. r=lsalzman

Because of the way the canvas 2D implementation juggles between path builders and paths, we have to keep track of current and first points in the path, in case a builder is created out of it.

Differential Revision: https://phabricator.services.mozilla.com/D33757

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extra : source : c79b3af2395894b449731fccdd9eb006b4d0189e