Fix test for "almost visible" row in GetFirstFullyVisibleRow()

The test didn't correspond to the comment and resulted in always
considering the first row not visible instead of the intended effect.

src/generic/grid.cpp

@@ -7843,7 +7843,7 @@ int wxGrid::GetFirstFullyVisibleRow() const
 
         // If the row is not fully visible (if only 2 pixels is hidden
         // the row still looks fully visible).
-        if ( GetRowTop(row) - 2 < y )
+        if ( GetRowTop(row) + 2 < y )
         {
             // Use the next visible row.
             for ( ;; )