QGraphicsView::mapFromScene() rounds coordinates, causing items at the edge to disappear on scaled displays

QGraphicsView::mapFromScene() returns a QPolygon, which implicitly rounds coordinates to integers.  
This causes visual artifacts where items near the edge of the view may be clipped or disappear,  especially on high-DPI displays where a single logical pixel may map to multiple physical pixels.

Steps to reproduce:
1. Create a QGraphicsView and QGraphicsScene.
2. Add an item (e.g., QGraphicsRectItem) at the scene's edge.
3. Scale the view (e.g., view->scale(1.1, 1.1) or view->scale(1/1.1, 1/1.1)).
4. Use mapFromScene() to get the mapped polygon and observe that its bounding rect may shift due to integer rounding.

Expected behavior:

• mapFromScene() should return a QPolygonF instead of QPolygon, preserving fractional values.  
• This would prevent unintended clipping issues.