Increase the default queue size. This helps with "missed" frames due to
the asynchronous nature of the API. All the other VOs are synchronous,
so if rendering and displaying takes a while, the common code in vo.c
will be blocked until it can continue. But with opengl-cb, vo.c might
immediately push the next ready frame, which causes the current frame
to be dropped _if_ it wasn't rendered yet.
One could fix this by making vo.c wait a while (until the API user calls
the render function, which pulls the frame). But setting the default
queue size to 2 seems much simpler: instead of dropping the frame, it
will be pushed to the API user once the next renderer call finishes.
(This is still a bit strange, and will hopefully be cleaned up when
video scheduling is redone, but for now this appears to deliver
relatively good results.)