`ensureSourceClockSet()` was intended to only run when the adjustable
source hasn't been set at all yet. As it turns out permitting it to run
unconditionally can break the state of the underlying interpolated
clock. This is caused by the following factors:
* While the decoupleable clock is running, its `CurrentTime` does not
come from either the source clock, or the internal stopwatch; it is
instead calculated using the base `InterpolatingFramedClock` logic.
* A source change of a decoupleable clock seeks the provided source
clock to the decoupleable's current time.
* When an interpolating clock is seeked (decoupleable clock is also
an interpolating one), its interpolation state
(`{Last,Current}InterpolatedTime`) are reset to 0.
* If the interpolating clock determines that its current time is too
far away from the source's time (which was set when the source is
changed), it will ignore the source and instead continue to use
its current time until the source clock has caught up.
Overall, the source change is not really necessary if a source is
already there. The only reason to ensure it was set was to make sure
the first seek of the gameplay clock wasn't performed in decoupled
mode. Therefore, add a guard to make sure the source is only set if
there isn't one already.
The UserFinishedPlaying event may trigger before the event is subscribed
to by SpectatorScreen. For such cases, an extra check is done to make
sure the user is _actually_ playing.
Uses the resolved working beatmap instead of resolving it every time.
Also uses the EditorBeatmap itself as playable beatmap, as it is of type `IBeatmap` already, and `.PlayableBeatmap` forwards everything anyway.
