ao_alsa: fix unpause path atfer previous commit

The resume code was accidentally fully removed from this code path.
vo_opengl_cb: always setup/break vertex array bindings

Originally, this code was written to have full control over the OpenGL
state, rather than having to cooperate with unknown components by being
embeded like vo_opengl_cb is meant to be. As a consequence, it was
thought to be ok to setup a global binding (if the context is below
OpenGL 3.0, which guarantees VAOs).

This could break badly. Fix it by setting up and breaking the bindings
on entry/exit.
vo_opengl_cb: use VAOs even in OpenGL 2.1

Using them reduces state change, since now at least vo_opengl_cb has to
setup/break the vertex array bindings on every frame if no VAOs are
available.

This reverts the VAO related change in commit f64665e7.