mesa: Create pointers for multithread marshalling dispatch table.

mesa: Create pointers for multithread marshalling dispatch table.

This patch splits the context's CurrentDispatch pointer into two
pointers, CurrentClientDispatch, and CurrentServerDispatch, so that
when doing multithread marshalling, we can distinguish between the
dispatch table that's being used by the client (to serialize GL calls
into the marshal buffer) and the dispatch table that's being used by
the server (to execute the GL calls).

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>