bug in screen-gamma handling

[Richard Stallman <rms@gnu.org>]

I investigated this bug report

    From: Oliver Scholz <alkibiades@gmx.de>
    Subject: Bug in current CVS w.r.t. `screen-gamma'
    Date: Sat, 21 Sep 2002 14:27:39 +0200

    * Eval this:

    (progn
      (push (cons 'screen-gamma 1.5) default-frame-alist)
      (set-face-foreground 'default "White")
      (set-face-background 'default "#0f1f4f"))

    * And hit `C-x 5 2'.

    Now in the new frame only the part of the buffer which contains text
    is displayed in the background-colour _modified_ by the
    gamma-value. The rest of the buffer is displayed in the unmodified
    (in this case: lighter) colour.

and I think the cause is that Emacs fails to use the gamma
to set the GC that is used by the primitive XClearArea.

Can anyone tell me how to set that GC?
What Xlib primitive does the job, and how does one use it?