The entrypoints take a mix of __DRIscreen * and void * (screen private)
arguments (similarly for contexts and drawables). This patch does away
with passing the private void pointer and always only passes the fully
typed __DRIscreen pointer and always as the first argument.
This makes the interface more consistent and increases type safety, and
catches a bug where we would pass a screen private to DRIdrawable::getSBC.
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