getHitShapeOnCanvasPointerDown

The shape under the pointer, as a click should resolve it.

Not the same question as editor.getShapeAtPoint, which answers about geometry. This one answers about *intent*, and differs in three ways that each come from how a click reads to a person:

- a filled shape is hit anywhere inside it, an unfilled one only on its edge, because clicking through the middle of an outline is how you select what is behind it; - a locked shape is not hit at all; - a frame is hit on its edge and its heading, not across its interior, so a click inside a frame reaches what is in the frame.

hitLabels widens it to count a shape's text label as part of the shape, which is what a double-click-to-edit needs.