title: QML canvas drawImage does not work properly with AA_EnableHighDpiScaling
body: <p>I create a qml application where is drawing on Wacom tablet to qml canvas.
  After drawing, I send the base64 image to the server and if I need it, I read it and draw it back.</p>
  
  <p>Drawed image look good. When i looked image on server, it is in good quality. But when i draw it again into canvas is little blurry. If i save and redraw image repeatly, it is more and more blurry.</p>
  
  <p>On HD screen it is not so visible, but when i use 4K screen, image quality is terrible.</p>
  
  <p>When i don't use Qt::AA_EnableHighDpiScaling is still in good quality. But without Qt::AA_EnableHighDpiScaling is application gui too small on 4k screen.</p>
  
  <p>When i disable smooth and antialising in canvas, image is not blurry but it not looks good.</p>
  
  <p>this i am use for save image:</p>
  
  <pre><code>var loadedImage = canvas.toDataURL("image/png")
  </code></pre>
  
  <p>this is for loading image :</p>
  
  <pre><code>canvas.loadImage(loadedImage)
  
  Canvas{
     id: canvas
     onImageLoaded:{
        var ctx = canvas.getContext("2d");
        ctx.drawImage(loadedImage,canvas.x,canvas.y,canvas.width,canvas.height);
        canvas.requestPaint();
  }
  </code></pre>
  
  <p>and part of drawing function:</p>
  
  <pre><code>var ctx = canvas.getContext("2d");
  ctx.beginPath();
  ctx.moveTo(canvas.prevX, canvas.prevY);
  ctx.lineTo(newPoint.x, newPoint.y);
  ctx.stroke();
  ctx.closePath();
  canvas.prevX = newPoint.x;
  canvas.prevY = newPoint.y;
  canvas.requestPaint();
  </code></pre>
  
  <p>How to fix drawImage when is AA_EnableHighDpiScaling enabled? I use Qt 5.9.</p>