[shaping] add Advance; deprecate some other fields

[benoitkugler]

This implements the transition path discussed in #210.

I've tried and make the minimal changes so that tests pass. As proposed, this PR

• adds methods Glyph.TextIndex() int, Glyph.RunesCount() int, Glyph.GlyphsCount() int
• adds a field Glyph.Advance
• deprecates XAdvance, YAdvance, ClusterIndex, RuneCount, GlyphCount fields

[whereswaldon]

Thank you! I look forward to the savings this will enable.

[benoitkugler]

@andydotxyz Any chance you could have a look at these changes? It would be nice to publish them and start updating users codebases.

[andydotxyz]

Sorry this escaped me in a holiday catchup.

Thanks for putting the effort into a migration path.

Perhaps we could/should now tag a release and note that it's the last one supporting these deprecated fields?