gallium/u_threaded: fix glMultiDrawElementsEXT() use-after-free and data corruption bugs
Fix for two bugs: a use-after-free bug due to wrong reference counting, and an index buffer data corruption bug, both triggerable with glMultiDrawElementsEXT()
when more than one TC_CALL_draw_multi
slot is allocated for one glMultiDrawElementsEXT()
call.
Backport to stable recommended. I think this bug has been lurking in Mesa for a very long time.