x11: Improve redraw of empty spaces.

x11: Improve redraw of empty spaces.

When the game area is showing only the middle portion with the empty
border, we can clear this with 4 or fewer draw calls instead of 16.
Let's handle this as a special case in the tile drawing code.

Moreover, we don't need to draw anything in these spaces on expose
events, as the exposed areas are already filled with the background.