docs: Update GL3.txt.
docs: Update GL3.txt.

We neglected to list the deprecation model/forward compatible context
support.

inverse() has been done for a while.

None of us know what "highp change" means; GLSL 1.30 already added the
ability to recognize precision keywords, and it doesn't look like 1.40
has any new requirements there (precision keywords still have no meaning).

Signed-off-by: Kenneth Graunke <bd2562f754ec92342f93f61c25d731e290a2ffa8@whitecape.org>

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