mesa/glthread: add glthread "perf" counters and pass them to gallium HUD

mesa/glthread: add glthread "perf" counters and pass them to gallium HUD

for HUD integration in following commits. This valuable profiling data
will allow us to see on the HUD how well glthread is able to utilize
parallelism. This is better than benchmarking, because you can see
exactly what's happening and you don't have to be CPU-bound.

u_threaded_context has the same counters.

Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>