Fix for EXT-7414 (Jumping to a specific time in a video stream does not work in Viewer 2.)
Fix for EXT-7414 (Jumping to a specific time in a video stream does not work in Viewer 2.)

This was a race condition.  In the repro case, LLViewerMediaImpl::seek() was getting called before the media source got a chance to be loaded (since the load is now delayed at least 1 frame due to the priority calculations), so the seek command was being dropped.

The fix is to save the seek time in mPreviousMediaTime in this case, and rearrange the code in LLViewerMediaImpl::loadURI() so that it gets used whenever it's non-zero.  I've also added similar code in LLViewerMediaImpl::pause() and LLViewerMediaImpl::start() to set mPreviousMediaState.  (If there are any cases where content wants parcel media to start out paused, they would have had a similar issue.)

Reviewed by Callum and Richard at http://codereview.lindenlab.com/2478002 .

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