Document::symbol_art_bounds measured a symbol's art without the
document, so an instance of another symbol in that art counted as the
20 x 20 square. It now measures the art through SymbolBoxes, so a nested
instance takes its own symbol's art, and an inline graphic of the outer
symbol is sized by it and follows it when that symbol changes.
- The inline resolve order also follows the instances that size a
symbol's art, so a symbol is measured after the type in the art of
the symbols it holds is laid out again.
- A symbol whose art reaches itself through an instance and an inline
graphic is an empty slot. Only instances that the measurement uses
count, so one a clipping path hides adds no dependency.
- A missing symbol, a symbol without art, a symbol that holds an
instance of itself, and nesting 32 deep still measure as the square.
- SymbolBoxes finds symbols through a name index built once, and the
inline resolve pass and inline_art_bounds share one index, instead
of scanning the symbol list for each lookup.