function BoundingBox() {
    this.x1 = Number.NaN;
    this.y1 = Number.NaN;
    this.x2 = Number.NaN;
    this.y2 = Number.NaN;
}
A bounding box is an enclosing box that describes the smallest measure within which all the points lie.
It is used to calculate the bounding box of a glyph or text path.

On initialization, x1/y1/x2/y2 will be NaN. Check if the bounding box is empty using `isEmpty()`.

@exports opentype.BoundingBox
@class
@constructor