[propagate_anchors] detect component cycles, fix infinite loop #1023
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a not too dissimilar configuration was found in https://github.com/googlefonts/genos
just like fontc effectively does; at least until we implement proper support for dereferencing components from special layers
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Fixes #1022
cyclical composites with infinite component depth are now correctly detected and skipped with a warning (they will probably just error out down the pipeline anyway).
We traverse the component trees in depth-first order while keeping track of the glyphs that we are currently visiting so we can detect cycles as they happen.EDIT: I now copied @cmyr's solution from googlefonts/fontc#907
We now keep track of the composites that are waiting for their components as we process the queue, as well as the length of the queue at that point, and if we encounter the same glyph twice without the queue having gotten smaller it means we aren't making any more progress, thus we have a cycle.
Also, since we currently can only resolve component refs between master layers, for now we skip processing the other layers (fontc propagate_anchors.rs similarly only does master layers, see #1017).