VIDEO: Make getFrameAtTime() public
VIDEO: Separate external and internal tracks

Prevents subclasses from having access to any audio track added from another file
VIDEO: Use the main isSeekable() in seekToFrame()
VIDEO: Improve accuracy of getFrameTime() and getFrameAtTime()
VIDEO: Add support for seeking in AVI videos with an index

Rewinding will work in any AVI video
Merge branch 'avi_seeking' into zengine-wip