Slightly simplify wxGrid refresh logic

Use new ShouldRefresh() helper instead of testing for !GetBatchCount()
before calling Refresh().

Also check for GetBatchCount() inside CalcDimensions() itself, which
means that it can now be called unconditionally.

No real changes.

include/wx/generic/grid.h

@@ -2454,6 +2454,12 @@ private:
     // release the mouse capture if it's currently captured
     void EndDraggingIfNecessary();
 
+    // return true if the grid should be refreshed right now
+    bool ShouldRefresh() const
+    {
+        return !GetBatchCount() && IsShownOnScreen();
+    }
+
 
     // return the position (not index) of the column at the given logical pixel
     // position