Workaround for flyTo bug

[hyperknot]

1. Made a test for checking the flyTo bug
2. Changed the old flyTo test to use a visible div, as it is required for flyTo animation.
3. Split the long line of r() inside flyTo to multiple lines.
4. Added workaround for floating point precision error in flyTo.
5. Changed the zoompan example to trigger this bug when clicking on DC.

This workaround now effectively limits the animation of flyTo to maximum 9 zoom levels, after that it jumps at the end. Still this is much better then making the browser go into an infinite loop.

Proper fix would be to change the r() function not to use a tiny number and then take it's logarithm.
cc: @mourner

[hyperknot]

#4226

[mourner]

I think there is a better workaround for this — we addressed this in GL JS some time ago. See this code https://github.com/mapbox/mapbox-gl-js/blob/master/js/ui/camera.js#L636-L645