Before there was no way of getting a white background despite having
the approriate code.

Backport of this commit:

  commit cb0a565d2b2cf8823abbd77b4426cc2237731dc1
  Author: Daniel Stone <daniel@fooishbar.org>
  Date:   Fri Aug 18 17:04:48 2006 +0300

    dix: add whiteroot flag
    Add a -wr option to use a white root window, and use a BackPixel rather
    than BackPixmap for both white and black root windows.

Fixes ArcticaProject/nx-libs#832