Inconsistent texture provider checks

_emphasized text_texture provider checks are implemented differently for different items (e.g. Image vs Canvas vs QQuickPaintedItem), leading to differences in behavior that appear to be unintentional.

As an example, if you take the code from https://doc.qt.io/qt-6/qml-qt5compat-graphicaleffects-fastblur.html#example and replace the Image with a QQuickPaintedItem, it will trigger "QQuickPaintedItem::textureProvider: can only be queried on the rendering thread of an exposed window", and the blur will not be applied.

To reproduce that without needing a custom subclass, add the following import and replace the Image with DialImpl:

import QtQuick.Controls.Fusion.impl

...

DialImpl { // replace Image with this
    id: bug
    anchors.fill: parent
}