Fixed map positioning issues in the World Tool

* Fixed an off-by-one error when adding the current map to a loaded world.
  The map was placed one pixel too much to the right.

* Fixed grid snapping when dragging a map to snap the target position
  rather than the offset. Also added handling of Control modifier to
  disable the grid snapping.

Closes #2970

(cherry picked from commit 8693dce)

src/tiled/abstractworldtool.cpp

@@ -361,7 +361,7 @@ void AbstractWorldTool::addToWorld(const World *world)
     // Position the map alongside the last map by default
     if (!world->maps.isEmpty()) {
         const QRect &lastWorldRect = world->maps.last().rect;
-        rect.moveTo(lastWorldRect.topRight());
+        rect.moveTo(lastWorldRect.right() + 1, lastWorldRect.top());
     }
 
     QUndoStack *undoStack = DocumentManager::instance()->ensureWorldDocument(world->fileName)->undoStack();

src/tiled/worldmovemaptool.cpp

@@ -192,16 +192,19 @@ void WorldMoveMapTool::mouseMoved(const QPointF &pos,
     }
 
     // calculate new drag offset
-    const MapRenderer *renderer = mDraggingMap->renderer();
-    const QPoint newOffset = renderer->screenToPixelCoords(pos - mDragStartScenePos).toPoint();
-    mDragOffset = renderer->pixelToScreenCoords(snapPoint(newOffset, mDraggingMap)).toPoint();
+    const QRect currentMapRect = mapRect(mDraggingMap);
+    const QPoint offset = (pos - mDragStartScenePos).toPoint();
+
+    QPoint newPos = currentMapRect.topLeft() + offset;
+    if (!(modifiers & Qt::ControlModifier))
+        newPos = snapPoint(newPos, mDraggingMap);
+
+    mDragOffset = newPos - currentMapRect.topLeft();
 
     // update preview
     mDraggingMapItem->setPos(mDraggedMapStartPos + mDragOffset);
     updateSelectionRectangle();
 
-    auto newPos = mapRect(mDraggingMap).topLeft() + mDragOffset;
-
     setStatusInfo(tr("Move map to %1, %2 (offset: %3, %4)")
                   .arg(newPos.x())
                   .arg(newPos.y())