function stride2offset( N, stride ) {
	if ( stride > 0 ) {
		return 0;
	}
	return ( 1 - N ) * stride;
}
Returns the index offset which specifies the location of the first indexed value in a strided array.

@param {NonNegativeInteger} N - number of indexed elements
@param {integer} stride - index increment
@returns {NonNegativeInteger} offset - offset

@example
var offset = stride2offset( 10, -10 );
// returns 90