Pop WebRender reference frames in layout 2020

[cbrewster]

Followup of #26063

I'm not familiar with layout 2020, but I think all we need to do is call pop_reference_frame at the end of the function if the following returns true:

servo/components/layout_2020/display_list/stacking_context.rs

Lines 477 to 478 in 687156a

	let established_reference_frame =
	self.build_reference_frame_if_necessary(builder, &border_rect);

cc @mrobinson @dralley

[mrobinson]

I think that pop_reference_frame is only necessary to adjust mapping of stacking context coordinates to reference frame coordinates. Since Layout 2020 only uses reference frame coordinates, it shouldn't be necessary to include this in the display list.

On the other hand, now that Layout 2020 is using reference frame coordinates everywhere, it raises the question of whether or not Layout 2013 and Gecko should as well. This would have the benefit of allowing the removal of the coordinate space mapper from scene_building.rs and removing the PopReferenceFrame display item. Spatial nodes are allowed to have children in various places in the display list, not just hierarchically so I have some doubts about the coordinate mapping anyhow. Thoughts @dralley?

The refactor that you are suggesting probably makes sense either way.

[dralley]

Ah, sorry, I deleted that comment after thinking about it some more. Old-layout passed around intermediate objects and I didn't immediately recognize that push_reference_frame was not using that strategy, and that the function call required a bunch of bits of intermediate information. Once I recognized that, the refactor idea didn't make quite as much sense, or at least would require a bit more work.

I can't make any judgements on big-picture strategy things.. I have maybe 20 hours of combined time looking at any of this code :)