mesa: Add infrastructure for a worker thread to process GL commands.

mesa: Add infrastructure for a worker thread to process GL commands.

v2: Keep an allocated buffer around instead of checking for one at the
    start of every GL command.  Inline the now-small space allocation
    function.
v3: Remove duplicate !glthread->shutdown check, process remaining work
    before shutdown.
v4: Fix leaks on destroy.
V5: (Timothy Arceri) fix order of source files in makefile

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>