With this change, a stacking context can be established by any
paintable, including inline paintables. The stacking context traversal
is updated to remove the assumption that the stacking context root is
paintable box.
Out-of-flow boxes (floating and absolutely-positioned elements) were
previously collected and put in the anonymous block wrapper as well, but
this actually made hit testing not able to find them, since they were
breaking expectations about tree structure that hit testing relies on.
After this change, we simply let out-of-flow boxes stay in their
original parent, preserving the author's intended box tree structure.
In general it is not safe to convert any arbitrary floating-point value
to CSSPixels. CSSPixels has a resolution of 0.015625, which for small
values (e.g. scale factors between 0 and 1), can produce bad results
if converted to CSSPixels then scaled back up. In the worst case values
can underflow to zero and produce incorrect results.
Grid specification https://www.w3.org/TR/css-grid-2/#z-order defines
special painting order for grid items which should be the same as for
defined for inline-blocks in CSS2.
