make selection box sub-tile aware

src/ui/WorldRenderer.h

@@ -54,12 +54,11 @@ struct WorldRenderFrame
     const SelectionController&     selection;
     const BuildModeController&     buildMode;
     const std::vector<ActiveBeam>& beams;
-    bool                           isBoxSelecting;
-    // Whether that box drag has passed the movement threshold that tells it apart
-    // from a click; until it has, the rectangle is not drawn (REQ-UI-MULTI-SELECT).
-    bool                           isBoxDragMoved;
-    QPoint                         boxStartTile;
-    QPoint                         boxCurrentTile;
+    // The box being dragged, in world coordinates and normalized, or nullopt when no
+    // drag is in progress — a press that has not passed the movement threshold is
+    // still a click and offers none (REQ-UI-MULTI-SELECT). It is the same rectangle
+    // the view selects by, so what is drawn and what is selected cannot disagree.
+    std::optional<QRectF>          boxWorldRect;
     bool                           isDebugDrawEnabled;
 };