Bug fix: Ensure that frames are still dropped even after a pipeline restart
Bug fix: Ensure that frames are still dropped even after a pipeline restart

If stbt's image analysis is slower than real time frames can get queued
up in the pipeline meaning that the frames we are operating on may be out
of date.  To minimise this latency we drop frames from a queue in the
GStreamer pipeline.

This commit fixes a bug introduced in b8d39d4 where this leaky behaviour
was not being applied after the pipeline was restarted.

martynjarvis/stb-tester,martynjarvis/stb-tester,wmanley/stb-tester,martynjarvis/stb-tester,wmanley/stb-tester,LewisHaley/stb-tester,LewisHaley/stb-tester,wmanley/stb-tester,wmanley/stb-tester,stb-tester/stb-tester,LewisHaley/stb-tester,martynjarvis/stb-tester,LewisHaley/stb-tester,wmanley/stb-tester,LewisHaley/stb-tester,LewisHaley/stb-tester,stb-tester/stb-tester,martynjarvis/stb-tester,martynjarvis/stb-tester,stb-tester/stb-tester,stb-tester/stb-tester,LewisHaley/stb-tester,martynjarvis/stb-tester