Auto merge of #16317 - eloycoto:issue6799, r=emilio

Fix #6799: set stacking_context_position correctly on fragment_border_iterator

Hey,

First of all, this is my first PR to Servo project and I'm learning Rust, so sorry if you see something that it's not correct. I did that as best as I know.

This PR fix the issue #6799; I tried all the corner cases that I can think about it and always get the right result and the same as other browsers.

Related to the build:

- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] These changes fix #6799

In the other hand, I added the test in the cssom folder, is where getBoundingClientRect  is defined, so I think that is the best place.

I'm sure that the line 122 can be better, but I didn't find a way to transform a Point2D from f32 to px in a easy way.

I'm here to listen to your recommendations and fix any issue.
Thanks!

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components/layout/display_list_builder.rs

@@ -2208,13 +2208,7 @@ impl BlockFlowDisplayListBuilding for BlockFlow {
         if state.clip_stack.is_empty() {
             return;
         }
-
-        let border_box = self.fragment.stacking_relative_border_box(
-            &self.base.stacking_relative_position,
-            &self.base.early_absolute_position_info.relative_containing_block_size,
-            self.base.early_absolute_position_info.relative_containing_block_mode,
-            CoordinateSystem::Parent);
-
+        let border_box = self.stacking_relative_position(CoordinateSystem::Parent);
         let transform = match self.fragment.transform_matrix(&border_box) {
             Some(transform) => transform,
             None => return,