Keep last drawn current scene when cancelling an animation frame.

Without this change, in browsers with requestAnimationFrame support,
if a second frame is generated before the first frame has been
rendered, the 'current' scene for the rendering is the new scene
from the frame that was never rendered, which means that changes from
that scene never get picked up.

Fixes Elm issue 189 - https://github.com/evancz/Elm/issues/189

Note of level of testing: I have only had time to do rudimentary testing to check that this fixes my testcase from Elm issue 189 - it would be good if someone had the time to test it more fully, particularly on a browser that doesn't support requestAnimationFrame.