Changed to use a simpler curve interpolate method

[wmerrifield]

This is a PR to add a simpler simpler .curve() function, rather than having to do a lot of boilerplate with implementing a full .handler() functions when doing transitions. i.e. you can just have functions like this:

float smoothstep(float x) {
    return x * x * (3.0f - 2.0f * x);
}

float easeInOutElastic(float x) {
    const float c5 = (2.0f * (float)M_PI) / 4.5f;

    if (x == 0.0f) return 0.0f;
    if (x == 1.0f) return 1.0f;

    if (x < 0.5f) {
        return -(powf(2.0f, 20.0f * x - 10.0f) *
                 sinf((20.0f * x - 11.125f) * c5)) * 0.5f;
    } else {
        return (powf(2.0f, -20.0f * x + 10.0f) *
                sinf((20.0f * x - 11.125f) * c5)) * 0.5f + 1.0f;
    }
}

in your local app code, which simply convert a 0-1 value of "fraction through the animation" into an output value which varies from 0 (i.e. the start value) to 1 (i.e. the end value), and the intermediate values are simply lerped using what amounts to the existing code.

The changed API is an additional .curve property on the Clay_TransitionElementConfig structure. This is in addition to the old .handler callback, but either one or the other must be specified for the animation to function.

so you can still do

.transition = {
        .handler = Clay_EaseOut, // or my_own_Handler_Function
        .duration = 0.3f,
        .properties = CLAY_TRANSITION_PROPERTY_POSITION,

But if you want a "smoothstep" animation (or springy animation), instead of having to copy a ~50 line function each time, you can just write a one-line function (as shown above), and insert that.

.transition = {
        .curve = smoothstep,
        .duration = 0.3f,
        .properties = CLAY_TRANSITION_PROPERTY_POSITION,