Don't blit individual rects of region when when scrolling backingstore

The QPlatformBackingStore::scroll() API takes a QRegion as input, but
we have no guarantee that the individual source and destination rects
of the region will not overlap each other when applying the scroll offset,
so we can't naively iterate the rects and call qt_scrollRectInImage for
each one.

The reason this didn't cause any issues in practice was that the QWidget
repaint manager was always passing in a single rect as the region.

On the other hand, the client has requested a scroll of the given
region, so it might assume any other part of the backing store is
preserved as is. Scrolling the bounding rect of the region violates
this assumption.

Amends 19ef76b0606621f189d3bc56549d200f2f5ebb25.

Pick-to: 6.2 6.3
Change-Id: I27934dd6685311c0b53ea2adb60fa5997e360f6c
Reviewed-by: Laszlo Agocs <laszlo.agocs@qt.io>

src/gui/painting/qrasterbackingstore.cpp

@@ -90,8 +90,8 @@ bool QRasterBackingStore::scroll(const QRegion &region, int dx, int dy)
     const qreal devicePixelRatio = m_image.devicePixelRatio();
     const QPoint delta(dx * devicePixelRatio, dy * devicePixelRatio);
 
-    for (const QRect &rect : region)
-        qt_scrollRectInImage(m_image, QRect(rect.topLeft() * devicePixelRatio, rect.size() * devicePixelRatio), delta);
+    const QRect rect = region.boundingRect();
+    qt_scrollRectInImage(m_image, QRect(rect.topLeft() * devicePixelRatio, rect.size() * devicePixelRatio), delta);
 
     return true;
 }

src/plugins/platforms/offscreen/qoffscreencommon.cpp

@@ -197,8 +197,8 @@ bool QOffscreenBackingStore::scroll(const QRegion &area, int dx, int dy)
     if (m_image.isNull())
         return false;
 
-    for (const QRect &rect : area)
-        qt_scrollRectInImage(m_image, rect, QPoint(dx, dy));
+    const QRect rect = area.boundingRect();
+    qt_scrollRectInImage(m_image, rect, QPoint(dx, dy));
 
     return true;
 }

src/plugins/platforms/windows/qwindowsbackingstore.cpp

@@ -182,8 +182,8 @@ bool QWindowsBackingStore::scroll(const QRegion &area, int dx, int dy)
         return false;
 
     const QPoint offset(dx, dy);
-    for (const QRect &rect : area)
-        qt_scrollRectInImage(m_image->image(), rect, offset);
+    const QRect rect = area.boundingRect();
+    qt_scrollRectInImage(m_image->image(), rect, offset);
 
     return true;
 }

src/plugins/platforms/xcb/qxcbbackingstore.cpp

@@ -488,8 +488,8 @@ bool QXcbBackingStoreImage::scroll(const QRegion &area, int dx, int dy)
         if (hasShm())
             preparePaint(destinationRegion);
 
-        for (const QRect &rect : scrollArea)
-            qt_scrollRectInImage(m_qimage, rect, delta);
+        const QRect rect = scrollArea.boundingRect();
+        qt_scrollRectInImage(m_qimage, rect, delta);
     } else {
         ensureGC(m_xcb_pixmap);