Scene player fixes and improvements

[WithoutPants]

Various scene player fixes and improvements:

• suppress logging of context cancelled errors during live transcode
• if scrubbing (ie mouse button is down while dragging the scrubber), then don't play the video. Makes this behaviour consistent with direct streaming
• debounce the source loading code during live transcoding. This means that fewer ffmpeg processes should be spawned while dragging across the scrubber
• video will now play from the position clicked on the scrubber instead of from the start/resume time when clicking the scrubber before the video is started