[HarfBuzz] Getting rid of the script enum

[Behdad Esfahbod]

Hi,

I've been thinking about getting rid of the script enum and use straight tags.
 The tags will be ISO 15924 codes as defined by Unicode in
PropertyValueAliases.txt (eg 'Arab', 'Latn', ...).  That is different from
OpenType script tags but not drastically so.

The downside of this is that a few tables and switches become a bit slower.
The upside is that the API would be independent of the Unicode version, plus,
we can pass OpenType script tags directly into the engine (if you so desire).

Comments?

behdad