Honor X11 'Expose' Events.

[clort79]

As far as I can see, SFML does not support X11 'expose events'

https://www.x.org/releases/X11R7.7/doc/libX11/libX11/libX11.html#Exposure_Events

This means all SFML apps will need to redraw their window constantly, consuming excessive cpu resources under X11, unless a compositing windowmanager is used that renders windows to an offscreen buffer.  We cannot guarantee such will be available.

Without such support, this is the result:

[Laurent]

This means all SFML apps will need to redraw their window constantly

Yes, it's even written in the documentation and tutorials, in big red letters

consuming excessive cpu resources under X11

"Excessive" is a little... excessive.

SFML is not a desktop UI framework, it's clearly not made for that.