Review support for different easing functions on blendTo

[davidmanzanares]

In blend, we allowed (a long time ago) the usage of cubic, linear and other interpolation methods.

[davidmanzanares]

To clarify this, apart from the space of the input of a blend (the famous CieLAB vs sRGB), it is important to use easing functions to improve eye candy. See https://easings.net/es

I recommend porting https://github.com/warrenm/AHEasing/blob/master/AHEasing/easing.c to GLSL. (C is very similar to GLSL and the library has a liberal license, although this is not a copy/paste).

Right now, there are a few easing functions in interpolators.js. Some of them ported from the mentioned library.

The current state is that Blend accepts a fourth parameter which is not documented, but it is used by blendToViz to perform cubic easing instead of linear (which I think it's a better default).

I think we should:

• [ ] Review and expose the easing API
• [ ] Port and expose most easing functions
• [ ] Show a nice example. This plays well for example with feature interactive styling.