Mesa (master): i965: Add ARB_occlusion_query support.

[Keith Whitwell]

> + * We use before and after counts when drawing during a query so that
> + * we don't pick up other clients' query data in ours.  To reduce overhead,
> + * a single BO is used to record the query data for all active queries at
> + * once.  This also gives us a simple bound on how much batchbuffer space is
> + * required for handling queries, so that we can be sure that we won't
> + * have to emit a batchbuffer without getting the ending PS_DEPTH_COUNT.

Eric,

Won't using a single BO undermine the ability to asynchronously issue
new queries and read the results of old ones?

IE. if I do something like:

-- start A
-- render 1
-- end A

... time passes ...

-- start B
-- render 2
-- end B
-- read results of A

Will the fact that B has been issued and refers to the same buffer as
A mean that when you want to read the results of A, you'll end up
waiting not just on render 1, but also render 2?

Keith