GTK L&F: JTable vertical grid lines aren't repainted when used with INSERT drop modes in RTL

When a JTable is in drop mode that supports inserting cols, and the table is in RTL component orientation, grid lines aren't repainted after being highlighted to show the drop location.

- Compile and run the attached TableTest.java
- Drag from the component labeled "Drag from here" into the table
- Drag between the two columns farthest to the right, to show the line indicating the drop location
- Release the mouse
- Notice that the grid line isn't repainted.