Adds support for overflowing stops

[yeldarby]

This PR lets you pass overflow: true to an exponential function to disable clamping and calculate a value based on the base.

Behavior is identical between defined stops; it only varies outside of the defined range.

Linear functions

For linear functions it will use the slope between the two stops on the end of the range (or a slope of 1 if only one stop is given) and project out the line.

For example, the slope here is 1/2:

var f = MapboxGLFunction({
    type: 'exponential',
    stops: [[10, 1], [12, 2]],
    base: 1,
    overflow: true
});

t.equal(f(1), -3.5);
t.equal(f(8), 0);
t.equal(f(9), 0.5);
t.equal(f(10), 1);
t.equal(f(11), 1.5);
t.equal(f(12), 2);
t.equal(f(13), 2.5);
t.equal(f(14), 3);
t.equal(f(20), 6);

Exponential functions

For exponential functions it will use the base passed and extrapolate out from the nearest end stop.

For example, a single stop passed with a base of 2 means that it will double for each whole-number increase and halve for each whole-number decrease:

var f = MapboxGLFunction({
    type: 'exponential',
    stops: [[12, 1]],
    base: 2,
    overflow: true
});

t.equal(f(1), 1/2048);
t.equal(f(5), 1/128);
t.equal(f(11), 0.5);
t.equal(f(12), 1);
t.equal(f(13), 2);
t.equal(f(15), 8);
t.equal(f(20), 256);

Launch Checklist

• briefly describe the changes in this PR
• write tests for all new functionality
• document any changes to public APIs -- I think this will need to be added to the style spec
• get a PR review

Spawned from a discussion with @lucaswoj in mapbox/mapbox-gl-js#3062

[lucaswoj]

I'm 👎 on the name "overflow"

overflow the generation of a number or some other data item that is too large for an assigned location or memory space.

The term that is typically used for this concept is "clamp" (for example c++ std::clamp, lodash clamp, Uint8ClampedArray)

[lucaswoj]

Can we use still use interpolate in this case? Nothing within interpolate requires that input is within inputLower and inputUpper.

If not, calculateOverflow will need to handle arrays, like interpolate

[mourner]

Closing as stale. Let me know if you want to continue working on the PR.