1. Remove requirement for TrackRenderer implementations to report
current position, unless they are time sources.
2. Expose whether renderers have media to play. The immediate benefit
of this is to solve the referenced GitHub issue, and also to only
display the appropriate Audio/Video/Text buttons in the demo app
for the media being played. This is also a natural step toward
multi-track support.
Github issue: #541
Propagate elapsedRealtimeUs to the video renderer. This allows
the renderer to calculate and adjust for the elapsed time since
the start of the current rendering loop. Typically this is <2ms,
but there situations where it can go higher (normally when the
video renderer ends up processing more than 1 output buffer in
a single loop).
Also made variable naming more consistent throughout the package.
