rqt_bag: skips time when resuming playback

[ablasdel]

Due to the backend of rqt_bag being the same as rxbag this bug most likely still exists.

Pressing space will resume a paused bag, but it does not resume playback at the right time. Instead, it appears that the time keeps running: If you pause the bag for 1 second, it skips one second when it resumes.

migrated from this trac ticket:
https://code.ros.org/trac/ros/ticket/3490