fix: fix memory leak when cursor buffer is null

The code was incorrectly deleting the QSGNode directly when the cursor
buffer is null, which could cause memory management issues. The proper
way to handle node removal in Qt Quick Scene Graph is to mark the node
as dirty with the DirtyNodeRemoved flag, allowing the scene graph to
properly manage the node's lifecycle.

Log:

Influence:
1. Check that scene graph nodes are properly managed

Author: zzxyb

waylib/src/server/qtquick/wquickcursor.cpp

@@ -575,7 +575,10 @@ QSGNode *WQuickCursor::updatePaintNode(QSGNode *node, UpdatePaintNodeData *)
 
     // Ignore the tp->proxy, Don't use tp->qwBuffer()
     if (!tp->buffer) {
-        delete node;
+        if (node) {
+            node->markDirty(QSGNode::DirtyNodeRemoved);
+        }
+
         return nullptr;
     }