gfxinfo: Poll framestats in a thread

Created by: mcgeagh

gfxinfo can be executed with an additional 'framestats' parameter that gives detailed timestamp information for the last 120 frames. Added a new parameter to expose this functionality. As this only captures the last 120 frames, to get all frame data across the workload we need to poll at least every 2 seconds (assuming 60Hz vsync) and append the data to the same file. New start and stop functions for doing this have been added to the workload class, similar to how tracing is currently handled.