mesa: Track the current vertex/element array buffers for glthread.

mesa: Track the current vertex/element array buffers for glthread.

We want to support glthread on GLES contexts with reasonable apps, and on
desktop for apps that use VBOs but haven't completely moved to core GL.
To do so, we have to deal with the "the user may or may not pass user
pointers to draw calls" problem.

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>