Add SetTabWidth to Setter (non-functional currently)

setter.go

@@ -18,6 +18,7 @@ type TypeSetter struct {
 	face           font.Face
 	width, height  int
 	wrap           bool
+	tabWidth       int
 	
 	minWidth          fixed.Int26_6
 	layoutBounds      image.Rectangle
@@ -215,6 +216,14 @@ func (setter *TypeSetter) SetHeight (heignt int) {
 	setter.height = heignt
 }
 
+// SetTabWidth sets the distance between tab stops in em spaces.
+func (setter *TypeSetter) SetTabWidth (tabWidth int) {
+	if setter.tabWidth == tabWidth { return }
+	setter.layoutClean = false
+	setter.alignClean  = false
+	setter.tabWidth = tabWidth
+}
+
 // Em returns the width of one emspace according to the typesetter's font, which
 // is the width of the capital letter 'M'.
 func (setter *TypeSetter) Em () (width fixed.Int26_6) {