Add Anchor.Opposite() func

geometry.go

@@ -579,6 +579,23 @@ func (anchor Anchor) String() string {
 	return anchorStrings[anchor]
 }
 
+var oppositeAnchors map[Anchor]Anchor = map[Anchor]Anchor{
+	Center:      Center,
+	Top:         Bottom,
+	Bottom:      Top,
+	Right:       Left,
+	Left:        Right,
+	TopRight:    BottomLeft,
+	BottomLeft:  TopRight,
+	BottomRight: TopLeft,
+	TopLeft:     BottomRight,
+}
+
+// Opposite returns the opposite position of the anchor (ie. Top -> Bottom; BottomLeft -> TopRight, etc.).
+func (anchor Anchor) Opposite() Anchor {
+	return oppositeAnchors[anchor]
+}
+
 // AnchorPos returns the relative position of the given anchor.
 func (r Rect) AnchorPos(anchor Anchor) Vec {
 	return r.Size().ScaledXY(V(0, 0).Sub(Vec(anchor)))