Remove weird scaling/distortion effects when resizing a Qt Quick Window

In Qt 6.0 beta (and earlier, when using the Direct 3D graphics backend), when resizing a Qt Quick Window faster than the renderer can update the window, the content is distorted and scaling up or down, giving a buggy user experience. Instead, the old area which was painted before the resize event should be aligned to the top left and the excess space should be filled with a neutral background until the resized window is rendered, like many other applications including MS Office and Chrome do. 

The attached video qtbug.mp4 demonstrates the bug by example of the "Qt Quick Controls Gallery" example project (Release build).

The attached video vlc.mp4 shows VLC player smoothly resized. This behavior would be the optimum, but for complex and computationally expensive GUIs this might not always be possible.

The attached video chrome.mp4 (slowed down so you can see the black portions) shows Google Chrome being resized. Although not as smooth as VLC player, it fills the excess space which is not yet rendered with black rather than weirdly shaking and resizing the content. This would be also fully acceptable. Notice that the UI parts on the top left (back/forward buttons) do not move one single pixel while the window is resized. This is what we need for QML windows to create a smooth user experience.

The videos were recorded on Windows 10 with a high-end GPU (Vega 64). The renderer is Direct 3D 11.