function alignCenter( width, line, len, fill ) {
	var space = width - len;
	var left = (space/2)|0;
	return fillLine( line, fill, left, space-left );
}
Center aligns a line of slide text.

## Notes

-   If provided a line of text with an odd number of characters, the aligned text is offset by one character to the left and thus not perfectly balanced.

@private
@param {PositiveInteger} width - slide width
@param {string} line - line text
@param {NonNegativeInteger} len - effective line length
@param {string} fill - fill character
@returns {string} aligned text