glretrace: Don't attempt to replay failed glX* calls.

This is particularly important when application attempts
glXCreateContextAttribsARB with different profiles, as described in
http://lists.freedesktop.org/archives/apitrace/2014-May/000950.html

This change is just a first stab.  There are more glX* calls (and
cgl*/wgl* calls too) that need to be updates accordingly.