using the aplpha channel in linux-fb??

[Benjamin <thronerbe gmx de>]

hi there

at the moment i am modifying the linux-fb backend to work with an 
ordinary memory chunk
to use the rendered data with OpenGL, the purpose of that is to use gtk 
as a GUI for games.

It works fine so far, except that the black partions of the screen are 
fully visible, what is not what i want.
i want the windows apear above the OpenGL rendered background, the 
solution seams to be the alpha-
channel of the off-screen texture, but even though my fb is 32 bit the 
"aplpha channel" if always zero.

what i want to know is, how can i force it to be, say 0xff, everywhere a 
window (or a widget or drawable)
is drawn and 0x00 everywhere where no window is.

thanks in advance

Benjamin