queue2: don't change global buffering state from within query handler

queue2: don't change global buffering state from within query handler

When a buffering query is handled it uses the get_buffering_percent()
function to get some statitics. Unfortunately this function also
calculates whether the queue should be buffering and adapts the
global queue2 state in case of state transitions from/to buffering
(including whether a buffering message was posted on the bus!).

This means that there is a race which can cause buffering messages
to never posted if the global state changes happen as a result of aa
query instead of resulting from bytes flowing in/out.

Spotted by Sjoerd Simons.

Change to only query state in get_buffering_percent() and update
state only in update_buffering().

https://bugzilla.gnome.org/show_bug.cgi?id=705332