function getSlideBackground( x, y ) {

		let slide = typeof x === 'number' ? getSlide( x, y ) : x;
		if( slide ) {
			return slide.slideBackgroundElement;
		}

		return undefined;

	}
Returns the background element for the given slide.
All slides, even the ones with no background properties
defined, have a background element so as long as the
index is valid an element will be returned.

@param {mixed} x Horizontal background index OR a slide
HTML element
@param {number} y Vertical background index
@return {(HTMLElement[]|*)}