servo: Merge #19699 - Correctly place images with background-position: center (from pyfisch:background-position-fix); r=emilio

Add a regression ref test.
See also #19680.

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- [x] There are tests for these changes

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Source-Repo: https://github.com/servo/servo
Source-Revision: 12a74de5a181c03852550d04632548011f8e7cb9

--HG--
extra : subtree_source : https%3A//hg.mozilla.org/projects/converted-servo-linear
extra : subtree_revision : 917c23a500ce0f8a31814a58d8f23be155c431cc

servo/components/layout/display_list_builder.rs

@@ -1581,7 +1581,7 @@ impl FragmentDisplayListBuilding for Fragment {
         let mut tile_size = compute_background_image_size(bg_size, bounds.size, intrinsic_size);
 
         let mut tile_spacing = Size2D::zero();
-        let own_position = bounds.size - intrinsic_size.unwrap_or(Size2D::zero());
+        let own_position = bounds.size - tile_size;
         let pos_x = bg_position_x.to_used_value(own_position.width);
         let pos_y = bg_position_y.to_used_value(own_position.height);
         tile_image_axis(