Respect apiversion in overrides

We should not use overrides from foreign apiversions.
Strongly-typed enums for ES 3.0 (WIP3)

Implemented sections: Texturing, Sampler Objects, Sampler Queries,
Texture Image Specification.
Added "reuse" directive for enums

Sometimes an enum may reuse the tokens of another enum verbatim
(possibly adding a few extra tokens.) The reuse directive simplifies
the handling of this case:
<enum name="Foo">
  <reuse enum="Bar" />
</enum>
Strongly-typed enums for ES 3.0 (WIP4)

Implemented sections: Texture Image Specification, Alternative Texture
Image Specification, Compressed Texture Images, Texture Parameters,
Manual Mipmap Generation and Enumerated Queries.