fallbackswitch: Fix draining of the backup pad.
fallbackswitch: Fix draining of the backup pad.

When not autoswitching between the primary and fallback pad, make sure
to drain the disabled pad to the current running time, and fix the
drain_pad_to_time() function to use the correct running time variable.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-rs/-/merge_requests/472>

sdroege/rsplugin,sdroege/rsplugin