[r2777] Graphical Glitch with the Chart of City List

[knightly]

When I open the city list's chart and close it it again, part of the chart remains in the blank area of the list. Moving the mouse pointer over these area will restore the correct graphics. But then if I open the chart again, there are blank holes in the chart. Moving the mouse pointer over these holes will again fix this.

This problem appears in both GDI and SDL versions of r2777.

[Dwachs]

works in rev 2778