Use transitions when using scroll wheel

[kkaefer]

We should smoothly animate the zooming action when using a mouse scroll wheel. When a new transition occurs while one is still in progress, we have to merge both transitions into one new transitions rather than queueing them.

[ansis]

added in f9b61f0

It works well for me, but it might be odd on different hardware. If its weird, re-open

When interrupting an in-progress transition, it calculates a new bezier that is continuous with the current position in the old one.

[kkaefer]

Looks very good, mouse scroll wheel zooming is definitely a lot smoother. I was thinking about implementing different zoom behavior when the user uses a trackpad: Since the user is touching directly, we should tie the zoom to the finger movement directly, i.e. not apply transitions in case of trackpad zooming. However, I'm not sure whether it's possible to distinguish trackpads from mousewheels in all browsers. Typically, the deltas for trackpad zoom are much lower, but they can get in the same area as mousewheel scroll deltas when the user zooms fast. Maybe we can do a heuristic detection that puts zooming into trackpad mode when the first couple of mousewheel events have a low (< 40 or so) delta, and in scrollwheel mode when it has high values.

[ansis]

Yeah, opened #206