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
Merge branch 'fix-cancelAnimationFrame-updates' of git+ssh://github.com/A1kmm/Elm into dev