Identifiers that are passed to FrameCallback functions (registered using add_frame_callback) that mark the progression of a frame in some way which usually means that new information will have been accumulated in the frame's corresponding FrameInfo object.
The last event that will be sent for a frame will be a COGL_FRAME_EVENT_COMPLETE event and so these are a good opportunity
to collect statistics about a frame since the FrameInfo should hold the most data at this
point.
<note>A frame may not be completed before the next frame can start so applications should avoid needing to collect all statistics for a particular frame before they can start a new frame.</note>