mesa: Add an attribute for conditions to turn off threading.

mesa: Add an attribute for conditions to turn off threading.

The threading for GL core is in place, but there are so few applications
actually using a core GL context that it would be nice to extend support
back.  However, some of the features of compat GL (particularly user
vertex arrays) would be so expensive to track state for that we want to be
able to disable threading when we discover that the app is using them.

Acked-by: Timothy Arceri <tarceri@itsqueeze.com>
Acked-by: Marek Olšák <maraeo@gmail.com>
Tested-by: Dieter Nützel <Dieter@nuetzel-hh.de>
Tested-by: Mike Lothian <mike@fireburn.co.uk>