servo: Merge #15334 - Fix the panic when transform is non-invertible …

…(from canaltinova:inverse); r=nox

<!-- Please describe your changes on the following line: -->
Fixes the panic when transform is non-invertible.
Counterpart of servo/webrender#823
r? @glennw

---
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `__` with appropriate data: -->
- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix #13266 (github issue number if applicable).

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- [x] There are tests for these changes OR
- [ ] These changes do not require tests because _____

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Source-Repo: https://github.com/servo/servo
Source-Revision: d2ae3d8bedf99c97877ec944d94f2aa72e67478d

--HG--
extra : subtree_source : https%3A//hg.mozilla.org/projects/converted-servo-linear
extra : subtree_revision : 8d73f7ee9d060bf2daca6cf6ed2f709a439d0164

servo/components/gfx/display_list/mod.rs

@@ -182,7 +182,14 @@ impl DisplayList {
             *client_point
         } else {
             let point = *translated_point - stacking_context.bounds.origin;
-            let inv_transform = stacking_context.transform.inverse().unwrap();
+            let inv_transform = match stacking_context.transform.inverse() {
+                Some(transform) => transform,
+                None => {
+                    // If a transform function causes the current transformation matrix of an object
+                    // to be non-invertible, the object and its content do not get displayed.
+                    return;
+                }
+            };
             let frac_point = inv_transform.transform_point(&Point2D::new(point.x.to_f32_px(),
                                                                          point.y.to_f32_px()));
             Point2D::new(Au::from_f32_px(frac_point.x), Au::from_f32_px(frac_point.y))