Starting new transition cancels running transition

The behavior of running a transition is janky and unpredictable,
when there is already a transition running on the same scene root.
Usually, the new transition simply jumps to the end values, or
jumps to the start values for that transition and animates from
there.

A better approach is to cancel any running transition first, the
start the new transition from that point.

Even better would be to blend old/new transitions, or at least adjust
the animation timing according to where/when the previous transition
stopped. In the meantime, this fix is at least better than the
previous approach of ignoring running transitions.

Change-Id: I4f5fabb55f6454f1e9d66589a9a7c36f9fc013fb