function enforce_size_divisibility([width, height], divisor) {
    return [
        Math.max(Math.floor(width / divisor), 1) * divisor,
        Math.max(Math.floor(height / divisor), 1) * divisor
    ];
}
Rounds the height and width down to the closest multiple of size_divisibility
@param {[number, number]} size The size of the image
@param {number} divisor The divisor to use.
@returns {[number, number]} The rounded size.