mechanism was replaced by drivers calling __glXScrEnableExtension from
their CreateScreen function.  All GLX extension state is track
per-screen.  This allows libGL to properly track GLX extension support
when there are multiple graphics cards in the system.

This necessitated several API and data-structure changes.  The
internal libGL API version was bumped to 20030813.

This work was a collaboration with Felix Ku"hling.