Add test for scrolling descendants of 'perspective-origin'

[Psychpsyo]

To spec and passes in every browser. Written while implementing this in ladybird.

[dbaron]

I'm a little surprised this isn't already in WPT, given that this interaction is important for parallax scrolling effects, which are possible only using the perspective property, specifically because it has a different interaction with the scroll offset than a perspective() transform function on the child.

One minor suggestion follows.

[dbaron]

I'd suggest also linking to https://drafts.csswg.org/css-transforms-2/#accumulated-3d-transformation-matrix-computation

[Psychpsyo]

No, because the accumulated 3D transform matrix does not apply here. It specifies the "final value of the transform used to render an element in a 3D rendering context".
And, according to the spec:

A 3D rendering context is established by a transformable element whose used value for transform-style is 'preserve-3d'

transform-style is not being tested here. This is purely about computing the correct perspective matrix for descendants of an element with transform and transform-origin when that element can be scrolled. So in essence, it's testing that the transform origin does not move together with the scrolling descendants, but rather stays fixed to the bounds of the scroll container that has perspective-origin.

I've changed the title of the test to something more like that, and also linked to https://www.w3.org/TR/css-transforms-2/#perspective-matrix-computation now, which is the relevant computation here.