[css-backgrounds] Algorithm for drawing the border image doesn't consider 'fill' keyword

[SebastianZ]

The algorithm described under Drawing the Border Image currently makes no difference whether the fill keyword of the of the border-image-slice property is set or not. In step 2 it is always talking about "top, middle and bottom" images.

It should be mentioned in the algorithm that in the case the fill keyword is not set, it only applies to the top and bottom images.

Sebastian

[fantasai]

Not sure it's really needed, but added a bullet to the Tile and Draw section about this anyway.