Comment 4 for bug 555641

Okay yeah... so what's going on there is that your hardware has a rendering buffer unit which supports textures up to a certain maximum size, which in the case of the GMA950 is only 2048x2048. Compiz renders the entire desktop as a single texture, so the max texture size is a limitation that affects dual-head cases.

A workaround is instead of putting the monitors side-by-side to make them one-above-the-other.

This won't be fixed at the distro level or in compiz (it's a hardware limitation), so I'm marking this wontfix. The solution will ultimately come when X.org upstream brings back Xinerama-mode so that you can have multiple screens each smaller than the max texture size, but stitched together in a way that allows multi-screen desktop layouts.

For more info see: http://wiki.compiz.org/FAQ#Root_window_is_larger_than_maximum_texture_size