make overlay buttons on call view clickable on chrome

src/platform/web/ui/css/themes/element/call.css

@@ -87,6 +87,10 @@ limitations under the License.
     display: flex;
     gap: 12px;
     margin-bottom: 16px;
+    /** Chrome (v100) requires this to make the buttons clickable
+     * where they overlap with the video element, even though
+     * the buttons come later in the DOM. */
+    z-index: 1;
 }
 
 .CallView_buttons button {