mesa/main: do not store unrecognized extensions in context

mesa/main: do not store unrecognized extensions in context

We process extension overrides only when we initialize the first
context, which means that unrecognized extensions only appear in the
first context created.

Let's instead store them in a global array, so we can apply them to all
contexts. This has the added benefit of making the initialization of the
first context less special, which allows us to clean up code a bit more.

Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4765>