OpenGL color depth problem

[mietkins]

Hey,
I have some problem with my color depth
I was enable DefaultDepth in xorg.conf to 16 bit
When i check desktop with xwininfo its ok have 16 bit colors but
when i run for example glxgears and check it with xwinifo its have 32 bit  
colors.
Problem is in all apps wich use OpenGL, all use 32 bit color depth
insted of 16 witch i enable in xorg.conf.
Next example: kde desktop effect don't work when i have 16 bit colors  
enabled in xorg.conf.
Any idea ??

System: KUbuntu 10.10
Graphics Card: Intel x3100
xserver-xorg: 7.5
mesa: 7.9
xserver-xorg-video-intel: 2.12

-- 
Greetings Martin