Remove use of stacking context local_bounds from make_polygon

[glennw]

It would be good to remove local_bounds from the StackingContext structure (and the associated public APIs).

It doesn't really have any meaning, now that we collect the bounding rect from the clip-scroll groups that make up a stacking context.

There's one piece of code that currently reads the local_bounds field in the make_polygon function.

Could we modify how that function works to not require the local bounds?

[glennw]

cc @kvark @mrobinson

[mrobinson]

@glennw I suppose this will be replaced by an offset?

[glennw]

@mrobinson Yes, that's what I was thinking.

[kvark]

It appears that we need to calculate local bounds specifically when traversing stacking context anyway, based on the children that are not ContextIsolation::Items.