TextMetrics advances aren't specified to reflect layout advances or paint advances

[litherum]

https://html.spec.whatwg.org/multipage/canvas.html#dom-textmetrics-advances

In WebKit, we have a distinction between layout advances (used for layout, and determining the bounds of each character when selecting using the mouse) and paint advances (delivered to the graphics subsystem when painting).

Which should we return in this API?

[annevk]

cc @whatwg/canvas