Boolean functions are incorrectly interpolated

[jfirebaugh]

When evaluated at 2, the function {stops: [[1, false], [3, true]]} should be false. But mapbox-gl-native currently evaluates it to true. This is because there's no explicit specialization for Interpolator<bool>, so the default formula a * (1.0 - t) + b * t is used.

[brunoabinader]

Great finding, @jfirebaugh!