qml performance fix, patch by Martin Tobias Holmedahl Sandsmark

Fixes #849

src/timeline2/view/qml/Clip.qml

@@ -92,7 +92,7 @@ Rectangle {
     signal trimmedOut(var clip, bool shiftTrim, bool controlTrim)
 
     onScrollStartChanged: {
-        clipRoot.hideClipViews = scrollStart > (clipDuration * timeline.scaleFactor) || scrollStart + scrollView.contentItem.width < 0
+        clipRoot.hideClipViews = scrollStart > (clipDuration * timeline.scaleFactor) || scrollStart + scrollView.width < 0
     }
 
     onIsGrabbedChanged: {

src/timeline2/view/qml/ClipThumbs.qml

@@ -60,7 +60,10 @@ Row {
                 Image {
                     id: thumbPlaceholder
                     visible: parent.running
-                    anchors.fill: parent
+                    width: parent.width
+                    height: parent.height
+                    sourceSize.width: width
+                    sourceSize.height: height
                     horizontalAlignment: Image.AlignLeft
                     fillMode: Image.PreserveAspectFit
                     asynchronous: true