The screenConfigs field of __DRIscreen points back to the containing
__GLXscreenConfigs struct. This is a serious abstraction violation; it
assumes that the loader is libGL and that there *is* a __GLXscreenConfigs
type in the loader.
Using the containerOf macro, we can get from the __DRIscreen pointer to
the containing __GLXscreenConfigs struct, at a place in the stack
where the above is a valid assumption. Besides, the __DRI* structs shouldn't
hold state other than the private pointer.
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