The purpose of Compositor::aboutToSwapBuffers() is notify the compositor
about a pending buffer. It's totally safe to call it multiple times if you
have multiple outputs. Ideally, it should also take an AbstractOutput or
screen id, but with the introduction of render loops per each output, we
won't need Compositor::aboutToSwapBuffers(). So, for the time being, this
change drops the annoying assert to make necessary per screen rendering
related refactorings easier to perform.