This brings the easing transition code a bit closer to how easings work
in gl-js. Instead of having an array of individual transitions for scale,
rotate, and pan, there is a single transition function that does all the
required calculations. This permits us to:
* Eliminate the "timeout" transition. (Fixes#126)
* Replace start/stopPanning() et al with setGestureInProgress(). Apps or
SDKs are expected to make paired calls to setGestureInProgress(). This
state will be ORed with the active easing state to determine when to
use texture interpolation. (Fixes#79)
* Run style recalculations only when an ease transition that affects the
zoom is in progress. (Fixes#1155)
