prepairing Qt6 usage

src/nodes/QskGraphicNode.cpp

@@ -50,7 +50,7 @@ void QskGraphicNode::setGraphic(
     const QskGraphic& graphic, const QskColorFilter& colorFilter,
     QskTextureRenderer::RenderMode renderMode, const QRectF& rect )
 {
-    bool isTextureDirty = ( QskTextureNode::textureId() == 0 );
+    bool isTextureDirty = isNull();
 
     QSize textureSize;
 

src/nodes/QskPaintedNode.cpp

@@ -34,7 +34,7 @@ QskPaintedNode::~QskPaintedNode()
 void QskPaintedNode::update(
     QskTextureRenderer::RenderMode renderMode, const QRect& rect )
 {
-    bool isTextureDirty = ( QskTextureNode::textureId() == 0 );
+    bool isTextureDirty = isNull();
 
     if ( !isTextureDirty )
     {

src/nodes/QskTextureNode.cpp

@@ -282,3 +282,10 @@ uint QskTextureNode::textureId() const
     Q_D( const QskTextureNode );
     return d->material.textureId();
 }
+
+
+bool QskTextureNode::isNull() const
+{
+    Q_D( const QskTextureNode );
+    return d->material.textureId() == 0;
+}

src/nodes/QskTextureNode.h

@@ -19,6 +19,8 @@ class QSK_EXPORT QskTextureNode : public QSGGeometryNode
     QskTextureNode();
     ~QskTextureNode() override;
 
+    bool isNull() const;
+
     void setRect( const QRectF& );
     QRectF rect() const;