make Arcball take vec2 instead of ivec2

[araid]

I often use Arcball to easily rotate 3D objects from the camera perspective. I call it from a variety of inputs other than the mouse, like touch, Leap motion or just keyboard.

The fact that mouseDrag takes an ivec2 causes small rotations to be choppy due to integer rounding, as you can see here: https://gist.github.com/araid/274bf3f2bc6b4ece2ea7
That would be fixed by using vec2 instead, and it wouldn't break anything.

In fact I would just rename all of its methods to get rid of the "mouse" part, since the class doesn't seem exclusive to mouse to me, but that's not really important.

[notlion]

Also, at least on OS X mouse position is represented as floats and inputs like graphics tablets set the cursor to sub pixel positions.

[axjxwright]

If this goes through, can it be applied to CameraUi as well?

[andrewfb]

This makes sense to me, and I think the CameraUi change would too. Anybody up for a PR to this effect?

[andrewfb]

Should be fixed by @lithium-snepo's commit above.