Add wxGrid::RefreshAttr() method to force attribute refresh.

A cached attribute may continue to be used even though the attribute returned
by a custom wxGridCellAttrProvider has changed so add a method to force wxGrid
to update the attribute by forgetting the cached copy.

Closes #12406.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@65451 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/generic/grid.h

@@ -1268,6 +1268,12 @@ public:
     void     SetRowAttr(int row, wxGridCellAttr *attr);
     void     SetColAttr(int col, wxGridCellAttr *attr);
 
+    // the grid can cache attributes for the recently used cells (currently it
+    // only caches one attribute for the most recently used one) and might
+    // notice that its value in the attribute provider has changed -- if this
+    // happens, call this function to force it
+    void RefreshAttr(int row, int col);
+
     // returns the attribute we may modify in place: a new one if this cell
     // doesn't have any yet or the existing one if it does
     //