Make ANW.setSwapInterval(0) work again

we can now queue/dequeue a buffer in asynchrnous mode by using the
async parameter to these calls. async mode is only specified
with those calls (it is not modal anymore).

as a consequence it can only be specified when the buffer count
is not overidden, as error is returned otherwise.

Change-Id: Ic63f4f96f671cb9d65c4cecbcc192615e09a8b6b