[shadow] Shadow font has wrong clusters

[behdad]

This long is wrong:
https://github.com/simoncozens/wasm-examples/blob/d0420534c95f2f2e1e9727c75f8769679e3a740d/shadow/src/lib.rs#L31

Try setting the cluster for the additional glyphs to the fluster of the last glyph.

Currently the font crashes eg. Pango.

[behdad]

(I tried building the font to fix it but my wasm-pack doesn't build.)