While in double buffering we only care about one previous presentation,
triple buffering will sometimes need to refer to the presentation from
two dispatches prior. So it might help to separate those frame stats
more clearly. This way each frame's dispatch and presentation times are
stored more cohesively such as to not be overwritten during overlapping
frame lifetimes.
Having two types of frame reference (dispatch and presentation) moving
at difference speeds meant that they could not be stored in a ring. Not
all dispatches become presentations and so storing them in a ring would
necessitate very complex conflict avoidance. Instead, a simple reference
counting model was used.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/3961>
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