When a session-range coincides with a loop-range location, moving
the session-range also updates the loop-range.
Keeping session and loop-range in sync can be useful if the whole
session is looped. However markers are treated individually, so
we need prevent invalid ranges.
If session-start and loop-start coincide, but loop-end is before the
end-marker, it is possible to move session-start beyond the loop-end.
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