x11: Fix redraw on window resize.

x11: Fix redraw on window resize.

It turns out that relying on expose events to trigger redraw of certain
tiles when enlarging is not actually a good idea.  No expose events will
be generated if the new portion of the window is obscured while resizing,
which means tiles along the bottom or right edge of the game/goal areas
may not get redrawn when they should be.

We can avoid the redundant drawing on resize another way.  Instead of
redrawing immediately in response to resize or expose events, we just
mark those tiles as needing update and only actually redraw after a
short delay, which is hopefully enough to catch the expose events
directly generated by the resize.