I figured out that drivers 169.12 and 100.14.19 doesn't crash xorg if I start an OpenGL game with compositor enabled.
With the 173.14.09 driver only fullscreen (with overlay rendering enabled in the compositor) mode works.
If I run cube windowed xorg dies, so for sure there's a regression in the new driver.
Regarding the Xinerama problem I couldn't find a regressen, it doesn't work with the older versions too
Since I have a triple head setup twinview isn't an option for me.
Possibly xorg itself has the bug. Any hints how to determine what the reason could be that xinerama doesn't work for me? (needless to say that I don't get anything out of xorg.log)