Normalize panning direction.

[birdinforest]

Do normalization on panning direction. I think this should be a propal practice to make panning works regardless scale of scene node and camera node.

In our case we need to scale down whole scene node to make it works with MapBox(Maybe it is.) Panning velocity changes if directions not been normalized.

[birdinforest]

I notice that there is build error from CircleCI for this PR. I don't know much about it, please let me know if I could fix some thing to make it works. Thanks.