1. SampleSource now has an explicit track selection state. This state is entered
after the source is prepared, and also by calling startTrackSelection.
2. endTrackSelection commits selection changes, and is responsible for doing the
right thing w.r.t starting/stopping/restarting load operations.
3. All sources now start or restart a load in the case of a new track selection.
This fixes a problem where a source could be advanced by repeatedly disabling
and re-enabling whilst paused. Some sources didn't restart a load in this case,
since the position was unchanged, however the downstream renderer would then
consume media up to the first keyframe in order to render something. Hence
each disable/re-enable would advance by a keyframe.
4. This change will enable a subsequent change where we'll discard media for
non-selected tracks earlier than we do currently (i.e. we'll hook the extractor
to a dummy track output, so the samples will never be written to a rolling
buffer). This will enable a further subsequent change where buffer contributions
are per-renderer rather than per-source.
Issue: #1041
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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=118024436
b3ce415e88