Start pop springs in the active state.

Summary: POP animations don't invoke the "did start" block until the turn of the run loop, which means our transition directors might appear to be at rest after describing a series of pop springs even though the springs will take effect soon. This diff ensures that the spring is represented as active immediately.

Reviewers: O2 Material Motion, #material_motion, O4 Material Apple platform reviewers, chuga

Reviewed By: O4 Material Apple platform reviewers, chuga

Subscribers: chuga

Tags: #material_motion

Differential Revision: http://codereview.cc/D2391

src/sources/popSpringSource.swift

@@ -69,6 +69,12 @@ private func configureSpringAnimation<T>(_ animation: POPSpringAnimation, spring
   animation.removedOnCompletion = false
   animation.velocity = spring.initialVelocity.read()
 
+  // animationDidStartBlock is invoked at the turn of the run loop, potentially leaving this stream
+  // in an at rest state even though it's effectively active. To ensure that the stream is marked
+  // active until the run loop turns we immediately send an .active state to the observer.
+
+  observer.state(.active)
+
   animation.animationDidStartBlock = { anim in
     observer.state(.active)
   }