This works just like masks, with a few more (spec imposed) limitations.
All the style properties on the contents of a <clipPath> are ignored,
and instead we just paint the "raw geometry" (as all black), then apply
that as an alpha mask to the target element.
If the element has both a `mask` and `clip-path` set, we compute the
masking area as the intersection of the mask rect and clip rect, then
apply the clip mask to the `mask` (producing a combined mask).
Fixes#19648Fixes#23006
8cec7ea221