mesa: Use atomics for shared VAO reference counts.

mesa: Use atomics for shared VAO reference counts.

VAOs will be used in the next change as immutable object across multiple
contexts. Only reference counting may write concurrently on the VAO. So,
make the reference count thread safe for those and only those VAO objects.

v3: Use bool/true/false for gl_vertex_array_object::SharedAndImmutable.

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