Fix TilingLayout::element_under position with global workspaces

I didn't see any issue with how mouse events were handled, but a bisect
showed b818a68 caused the issue.
Reverting the definition of `element_under` to the version before that
change

Comparing what both versions return, the right element is returned, but
the location returned is wrong. This makes the return value match the
position that was returned by the previous implementation. It seems to
be working correctly now.

Fixes #161.

src/shell/layout/tiling/mod.rs

@@ -3055,7 +3055,7 @@ impl TilingLayout {
                         mapped.is_in_input_region(&test_point).then(|| {
                             (
                                 mapped.clone().into(),
-                                last_geometry.loc - output_data.location - mapped.geometry().loc,
+                                last_geometry.loc + output_data.location - mapped.geometry().loc,
                             )
                         })
                     }
@@ -3086,7 +3086,8 @@ impl TilingLayout {
                                 orientation,
                             }
                             .into(),
-                            last_geometry.loc - output_data.location
+                            last_geometry.loc
+                                + output_data.location
                                 + tree
                                     .children(&id)
                                     .unwrap()