main/extensions: Add early extension override structures

During the early one_time_init phase of context creation, we
initialize two global gl_extensions structures.

We read the MESA_EXTENSION_OVERRIDE environment variable, and store
positive and negative overrides in two structures:
* struct gl_extensions _mesa_extension_override_enables
* struct gl_extensions _mesa_extension_override_disables

These are filled before the driver initializes extensions and
constants, therefore the driver can make adjustments based on the
desired overrides.

This can be useful during development of a new extension where the
extension is only partially ready. The driver can't actually advertise
support for the extension, but if it sees that the override is set for
the extension, then it can expose more supported parts of the
extension, such as upgrading context constants.

Signed-off-by: Jordan Justen <[email protected]>
Reviewed-by: Anuj Phogat <[email protected]>

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