Avoid white flashes when the display colors are inverted

Paint the window background white and hope the user has an overally
light UI theme, so the default window background is roughly the same
color as the theme's.

This fixes white flashes visible with Compiz 0.9 and inverted colors.
For windows with no alpha channel the current state of not filling the
background at all will result with effectively a "capture" or the
display under the window.  Thus, the window content will be the
effective displayed content after inversion, and thus an inversion of
that window's content will result in a "double inversion", leading to
maximum contrast and thus a "white flash" when using a light theme
(which becomes dark after inversion).

Filling the window with a plain color close to the theme's one will
prevent this effect.