x11 backend: Do not set a back pixel on windows
x11 backend: Do not set a back pixel on windows

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 <346c0ad49ac8427b61b3b55abbe37dc0ebd524f2@znc.in>

ascent12/wlroots,swaywm/wlroots,SirCmpwn/wlroots,swaywm/wlroots,ascent12/wlroots