servo: Merge #7912 - Make sure to correctly destroy old layers in collect_old_layers_for_pipeline (from eefriedman:compositor-layer); r=mrobinson

The previous code would skip calling clear_all_tiles(), which led to a
panic in rust-layers. ("You should have disposed of the pixmap properly
with destroy()! This pixmap will leak!")

Ran into this messing around with pdf.js; no minimized testcase.
Maybe related to #7895.

r? @pcwalton

Source-Repo: https://github.com/servo/servo
Source-Revision: 217c7da41304142ffaf7c02a3eb888b0f150c59d

servo/components/compositing/compositor_layer.rs

@@ -472,7 +472,9 @@ impl RcCompositorLayer for Rc<Layer<CompositorData>> {
                     }
 
                     // Keep this layer if it exists in the new layer list.
-                    return new_layers.iter().any(|properties| properties.id == extra_data.id);
+                    if new_layers.iter().any(|properties| properties.id == extra_data.id) {
+                        return true
+                    }
                 }
 
                 if let Some(ref subpage_info_for_this_layer) = extra_data.subpage_info {