playbin2: Make it possible for READY->PAUSED to succeed after it failed the first time
If READY->PAUSED failed in the source element we would've swapped the current and next group already. To allow READY->PAUSED to succeed after the first failure we have to swap the current and next group back again. This also ensure that we're again in the same state as before the failed state change and not at the next group. This was especially a problem for playbin2 pipelines that use the new mounting support in giosrc as the source would fail for READY->PAUSED the first time, the application mounts the location and then tries to go READY->PAUSED again (and this time it would succeed). Fixes bug #588078.