Redefine camera scroll as view center

src/ui/GameWorldView.h

@@ -134,6 +134,9 @@ private:
 
     float    tilePx() const;
     float    viewportWidthTiles() const;
+    // World-X (tiles) at the left edge of the viewport. m_scrollXTiles stores the
+    // view center; this derives the left edge the world<->widget conversions need.
+    float    viewLeftTiles() const;
     QPointF  worldToWidget(QVector2D worldPos) const;
     QPointF  tileToWidget(QPoint tile) const;
     QPoint   widgetToTile(QPoint widgetPt) const;
@@ -218,6 +221,7 @@ private:
     std::mt19937  m_rng;
     double        m_gameSpeedMultiplier;
     double        m_prevNonZeroSpeed;
+    // World-X (tiles) at the center of the viewport (see viewLeftTiles()).
     float         m_scrollXTiles;
 
     QTimer* m_renderTimer;