Better mapping of Blender curves to Calcium curves

[io7m]

Right now, the mapping is:

Blender  | Calcium
---------------------------------------
CONSTANT | CURVE_INTERPOLATION_CONSTANT
LINEAR   | CURVE_INTERPOLATION_LINEAR
EXPO     | CURVE_INTERPOLATION_EXPONENTIAL

The problem is that the obvious definition of linear is:

linear :: Double -> Double -> Double
linear x0 x1 factor = (x0 * (1 - factor)) + (x1 * factor);

... and therefore the obvious definition in Calcium for "exponential" is:

exponential :: Double -> Double -> Double
exponential x0 x1 factor = linear x0 x1 (factor * factor)

Unfortunately, this gives results that are more like QUADRATIC in Blender. In order to yield results that are more like Blender's idea of EXPO, the definition looks more like:

exponential :: Double -> Double -> Double
exponential x0 x1 factor = linear x0 x1 (factor * factor * factor * factor * factor * factor * factor * factor)

The actual Blender definitions are given in:

https://developer.blender.org/diffusion/B/browse/master/source/blender/blenlib/intern/easing.c$252

At the very least, Calcium's EXPONENTIAL should probably match Blender's EXPO, and the previous definition for EXPONENTIAL should probably be renamed QUADRATIC to match Blender's idea of quadratic.