Transition ratio getter function not correct for interrupted transitions.

The transition ratio getter simply uses ElapsedTime/Crossfade for the alpha on the transition.

Normally this is sufficient, although if you blend between two states A and B such that you interrupt the first blend and transition back A -> B -> A this will give incorrect values as we're already part way through the transition.

We essentially need the query alpha from the FAlphaBlend update on the transition. Unfortunately all the data we need to identify active transitions is editor only, so we need to expose that so we can go from a transition descriptor to any active transitions for that descriptor.

1) Create an animation blueprint with a state machine containing 2 states
2) Add a transition from each state to the other state
3) Add 2 bools and set them to trigger the transitions
4) Start the first transition and interrupt it with the second