Large Text: populate only lines visible in Window into SG

Text is rendered top-to-bottom: QQuickTextNode::addTextLayout()
iterates lines of text.  Now it first maps the window size() rectangle
into the Text item's coordinate system, and treats that as the viewport:
lines whose y coordinates fall outside are omitted.
This saves a huge amount of time and memory when the text is large.
So Flickable { Text { } } can actually be used in an ebook reader now,
for example.

QQuickItemPrivate::transformChanged(parent) is now used to inform
all children when a parent moves; so e.g. when a content item is
moved inside a "viewport" item (like a Flickable, or any other item
that is being used as a clipping viewport or just as a holder),
all children of the content item will be notified; and QQuickTextPrivate
uses this occasion to mark itself dirty so that the updatePaintNode()
will be called again.  This happens directly after the actual movement:

  QQuickTextPrivate::transformChanged  qquicktext.cpp        2990
  QQuickItemPrivate::transformChanged  qquickitem.cpp        5200
  QQuickItemPrivate::dirty             qquickitem.cpp        6339
  QQuickItem::setY                     qquickitem.cpp        6830
  QQuickFlickablePrivate::setViewportY qquickflickable.cpp   1800

Task-number: QTBUG-60491
Task-number: QTBUG-90734
Change-Id: I152d23caa725f194a186a604fbc8d0c07f597b16
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@qt.io>

1 files changed, 3 insertions, 3 deletions

diff --git a/src/quick/items/qquickflipable.cpp b/src/quick/items/qquickflipable.cpp
index a338985622..27fb02987c 100644
--- a/src/quick/items/qquickflipable.cpp
+++ b/src/quick/items/qquickflipable.cpp
@@ -70,7 +70,7 @@ class QQuickFlipablePrivate : public QQuickItemPrivate
 public:
     QQuickFlipablePrivate() : current(QQuickFlipable::Front), front(nullptr), back(nullptr), sideDirty(false) {}
 
-    void transformChanged() override;
+    void transformChanged(QQuickItem *transformedItem) override;
     void updateSide();
     void setBackTransform();
 
@@ -219,7 +219,7 @@ QQuickFlipable::Side QQuickFlipable::side() const
     return d->current;
 }
 
-void QQuickFlipablePrivate::transformChanged()
+void QQuickFlipablePrivate::transformChanged(QQuickItem *transformedItem)
 {
     Q_Q(QQuickFlipable);
 
@@ -228,7 +228,7 @@ void QQuickFlipablePrivate::transformChanged()
         q->polish();
     }
 
-    QQuickItemPrivate::transformChanged();
+    QQuickItemPrivate::transformChanged(transformedItem);
 }
 
 void QQuickFlipable::updatePolish()