Queues view crashes

If the Queues views was visible, and certain actions were taken while FFRend was paused, resuming would likely cause a crash. Some specific scenarios:

1) Pause, load a project with (a lot) less plugins, and then resume.
2) If at least one plugin has helpers: Pause, delete some plugins (but not the one with helpers), and then resume.

In all cases the issue was the Queues view accessing non-existent frames due to stale frame pointers. One case was in Renderer: though m_CurFrame was cleared in Run's stop case, the stop code wasn't executing, due to Run's erroneous initial no-op test (since removed). PluginHelper wasn't invalidating ANY of its frame pointers, neither input nor output.

Invalidation was not only lacking in places but also inconsistent in terms of timing, e.g. Plugin was clearing its input frame pointers on engine START (in ResetQueues), but clearing its output frame pointer on plugin STOP. There's no need to invalidate on stop, because the pointers don't actually become invalid until RunInit deletes the excess frames (in AllocFrames). Thus all frame pointer invalidation is now consistently done on engine START instead of engine STOP.

Plugin invalidates its input and output frame pointers in ResetQueues. PluginHelper does the same in ResetState. Renderer invalidates its current frame pointer in Run's start case, just before launching the worker thread.