`GlyphBuffer.serialize` should accept format flags

[hasanzakeri]

hr_shape::shape() exposes --no-glyph-names, --no-clusters, --no-positions, --no-advances, --glyph-extents, --glyph-flags via SerializeFlags. Our object API hardcodes SerializeFlags::default().

For Phase 5 this is fine — default() matches the format used by harfrust.shape() so the parity test holds. But it means the object API is strictly less expressive than the string API.

Options:

1. Accept a string flag list (gbuf.serialize(font, "no-clusters,no-positions"))
2. Accept boolean kwargs (gbuf.serialize(font, no_clusters=True, ...))
3. Add a SerializeFlags Python class wrapping the bitflags

Option 2 is the most Pythonic for an API surface this small.