Add api to QgsVectorTileRawData to handle situation where

replacement tiles from lower zoom levels have been used, and
where the tile ID to use for the data's clipping region
does not match the tile ID to be used when decoding geometries

src/core/vectortile/qgsvectortileloader.h

@@ -39,10 +39,21 @@ class CORE_EXPORT QgsVectorTileRawData
   public:
     //! Constructs a raw tile object
     QgsVectorTileRawData( QgsTileXYZ tileID = QgsTileXYZ(), const QByteArray &raw = QByteArray() )
-      : id( tileID ), data( raw ) {}
+      : id( tileID ), tileGeometryId( tileID ), data( raw ) {}
 
     //! Tile position in tile matrix set
     QgsTileXYZ id;
+
+    /**
+     * Tile id associated with the raw tile data.
+     *
+     * This may differ from the tile id in the situation where lower zoom level tiles have been used to replace
+     * missing higher zoom level tiles. In this case, the tileGeometryId should be used when decoding tiles
+     * to features in order to obtain correct geometry scaling and placement, while the actual tile id
+     * should be used when determining the region of the tile for clipping purposes.
+     */
+    QgsTileXYZ tileGeometryId;
+
     //! Raw tile data
     QByteArray data;
 };

src/core/vectortile/qgsvectortilemvtdecoder.cpp

@@ -42,7 +42,7 @@ bool QgsVectorTileMVTDecoder::decode( const QgsVectorTileRawData &rawTileData )
   if ( !tile.ParseFromArray( rawTileData.data.constData(), rawTileData.data.count() ) )
     return false;
 
-  mTileID = rawTileData.id;
+  mTileID = rawTileData.tileGeometryId;
 
   mLayerNameToIndex.clear();
   for ( int layerNum = 0; layerNum < tile.layers_size(); layerNum++ )