function strides2offset( shape, strides ) {
	var offset;
	var ndims;
	var i;

	ndims = shape.length;
	offset = 0;
	for ( i = 0; i < ndims; i++ ) {
		if ( strides[ i ] < 0 ) {
			// Note that, since the stride is negative, this operation increments, not decrements, the offset...
			offset -= strides[ i ] * ( shape[ i ]-1 );
		}
	}
	return offset;
}
Returns the index offset which specifies the location of the first indexed value in a multidimensional array based on a stride array.

@param {NonNegativeIntegerArray} shape - array shape
@param {IntegerArray} strides - stride array
@returns {NonNegativeInteger} offset - offset

@example
var shape = [ 2, 3, 10 ];
var strides = [ 30, -10, 1 ];

var offset = strides2offset( shape, strides );
// returns 20