Comment # 18 on bug 79850 from Sylvain BERTRAND

Ok... I did a bit of gdb.

It's a bloody mess, many things are optimized out and breakpoints are not
working consistently.

I got the DRI2CanFlip function to return FALSE for those lines:
dri2.c:934 and dri2.c:945

That said, I was unlucky about can_flip debugging, since I hit only the TRUE
returning code paths of DRI2CanFlip.