[css-filter-effects] Clarify the `filter region` for CSS reference filters

[smfr]

The Filter Effects spec describes the filter region in terms of how it applies to SVG elements, but its behavior on CSS boxes is not well defined.

In particular, if the CSS box to which the filter is being applies has ink overflow, I can't find a definition of how the filter region is computed. The behavior that Gecko and Blink have is to use the ink overflow bounds for userSpaceOnUse, and border box for objectBoundingBox, but I don't believe that's actually written down.

Using the ink overflow box is also a little surprising, because it's not precisely defined. E.g. UAs differ on the exact extent of box-shadow, for example.

Basic test that has different behavior in WebKit and other browsers: https://codepen.io/smfr/pen/WbwJBNr

I intend to make WebKit match other browsers, but I think the spec needs edits to make the behavior explicit.

[schenney-chromium]

My guess is that for userSpaceOnUse Blink is using the internal concept of the paint invalidation box, which is basically ink overflow. I agree with the point that this concept is not defined, and #8649 was closed WontFix.

On the one hand I worry about web-compat issues in changing the behavior. On the other hand I would be happy with always using the border box given that authors can explicitly extend the region if necessary to enclose ink overflow.

[smfr]

In addition, the filter property applies to "All elements", so we need to define what the reference box is for inline boxes.