Description of problem:
With Matrox MG550 a HW rendering is enabled by default:

$ glxinfo | grep render
direct rendering: Yes
OpenGL renderer string: Mesa DRI G400 AGP 1x

Thus System->Preferences->Desktop Effects, does allow to enable Compiz. What appears is heavily distorted, without window decoration. Parts of Compiz are working as one can tell by the menu appearance effect etc.

Trying to enable Compiz directly leads to:
$ compiz --replace
compiz (core) - Fatal: GLX_EXT_texture_from_pixmap is missing
compiz (core) - Error: Failed to manage screen: 0
compiz (core) - Fatal: No manageable screens found on display :0.0

I make no illusions of Compiz actually being supported by or supporting the MGA driver, but if that is the case, one should not be able to enable it through the graphical 'Desktop effects' settings

Version-Release number of selected component (if applicable):
xorg-x11-drv-mga-1.4.13-3.el6 

How reproducible:
Allways

Steps to Reproduce:
Described ^^^
  
Actual results:
Compiz appears distorted and unusable

Expected results:
Enabling desktop effects is not allowed, or compiz working as expected

Additional info:
screenshot will be attached