A display list should not contain coordinates shifted by scroll offset.
Instead, "scroll frame id" needs to be used. In the future it's going to
allow us reuse a display list in cases when only scroll offsets need to
be updated.
Before this change AddClipRect was a "special" because it didn't respect
scroll frame offset and was meant to be recorded using viewport-relative
coordinates. The motivation behind this was to record a "final" clip
rectangle computed by intersecting all clip rectangles used by a clip
frame. The disadvantage of this approach is that it blocks us from
implementing an optimisation to reuse display list if the only change is
in the scroll offset, because any scroll offset change leads to
invalidating all AddClipRect items within a list.
This change aligns AddClipRect with the rest of display list items by
making it account for scroll frame offset. It required discontinuing
the recording of the intersection of all clip rectangles within a clip
frame and instead producing an AddClipRect for each of them.
A nice side effect is the removal of code that shifts clip rectangle by
`enclosing_scroll_offset()` in a bunch of places, because now it happens
automatically in `DisplayList::apply_scroll_offsets()`.
Overflow clipping is currently implemented as:
1. Create clip frame for each box with hidden overflow
2. Calculate clip rect for each clip frame by intersecting padding boxes
of all boxes with hidden overflow in containing block chain
3. Assign enclosing clip frame (closest clip frame in containing block
chain) to each PaintableBox
4. Apply clip rect of enclosing clip frame in Paintable::before_paint()
It breaks when any CSS transform other than simple translation is lying
between box with hidden overflow and a clipped box, because clip
rectangle will be applied when transform has already changed.
The fix is implemented by relying on the following rule:
"For elements whose layout is governed by the CSS box model, any value
other than none for the transform also causes the element to establish
a containing block for all descendants."
It means everything nested into a stacking context with CSS transform
can't escape its clip, so it's safe to apply its clip for all children.
