Improve multi-head detection for OpenGLIsUnsafe

This was currently basically broken:
* Screen number got always attached
* openGLIsBroken did not check for screen number
-> KCM reported "everything is fine" while it wasn't

Now changed to:
* only attach screen number if it is a multi-head setup
* use same logic in both Composite and CompositingPrefs

Still problematic:
* kcm is not multi-head aware so it will report everything is fine in
case of a broken multi-head setup

REVIEW: 110631