Fix missing file from previous commit

src/timeline2/view/qml/MixShape.qml

+import QtQuick 2.11
+import QtQuick.Shapes 1.11
+
+Shape {
+    anchors.fill: mixBackground
+    asynchronous: true
+    opacity: 0.4
+    ShapePath {
+        fillColor: "#000"
+        strokeColor: "transparent"
+        PathLine {x: 0; y: 0}
+        PathLine {x: mixCutPos.x; y: mixBackground.height}
+        PathLine {x: 0; y: mixBackground.height}
+        PathLine {x: 0; y: 0}
+    }
+    ShapePath {
+        fillColor: "#000"
+        strokeColor: "transparent"
+        PathLine {x: mixBackground.width; y: 0}
+        PathLine {x: mixBackground.width; y: mixBackground.height}
+        PathLine {x: mixCutPos.x; y: mixBackground.height}
+        PathLine {x: mixBackground.width; y: 0}
+    }
+}