mesa: Avoid ABA problem on buffer object bind.

mesa: Avoid ABA problem on buffer object bind.

Make sure we do not run into the classic ABA problem on buffer object bind,
reusing this name and may be never rebind since we get an new name
that was just deleted and never rebound in between.
The explicit rebinding to the debault object in the current context
prevents the above in the current context, but another context
sharing the same objects might suffer from this problem.

Minor var renaming and comments edited by Brian.

Signed-off-by: Mathias Fröhlich <Mathias.Froehlich@web.de>
Signed-off-by: Brian Paul <brianp@vmware.com>