Before this commit, the x11 server would fill any exposed area with
white before the wlroots x11 backend got a chance to do anything. This
was e.g. visible when running rootston and resizing the window: When the
window becomes larger, the new area is filled with black.
By just not setting a back pixel value, this commit gets rid of this
behaviour.
Signed-off-by: Uli Schlachter <psychon@znc.in>
52b058c2a3