Fix child boxes rendering on top of borders

Closes #4

internal/system/box.go

@@ -75,7 +75,8 @@ func (this *System) newBox (outer anyBox) *box {
 		if box.parent == nil { return nil }
 		parentCanvas := box.parent.getCanvas()
 		if parentCanvas == nil { return nil }
-		return parentCanvas.SubCanvas(box.bounds)
+		drawableArea := box.bounds.Intersect(box.parent.getInnerClippingBounds())
+		return parentCanvas.SubCanvas(drawableArea)
 	})
 	if outer == nil {
 		box.drawer = box

internal/system/containerbox.go

@@ -298,6 +298,10 @@ func (this *containerBox) getCanvas () canvas.Canvas {
 	return this.canvas.Value()
 }
 
+func (this *containerBox) getInnerClippingBounds () image.Rectangle {
+	return this.innerClippingBounds
+}
+
 func (this *containerBox) notifyMinimumSizeChange (child anyBox) {
 	this.invalidateMinimum()
 	size   := child.minimumSize()

internal/system/hierarchy.go

@@ -193,6 +193,10 @@ func (this *Hierarchy) getCanvas () canvas.Canvas {
 	return this.canvas
 }
 
+func (this *Hierarchy) getInnerClippingBounds () image.Rectangle {
+	return this.canvas.Bounds()
+}
+
 func (this *Hierarchy) getModifiers () input.Modifiers {
 	return this.modifiers
 }

internal/system/internal-iface.go

@@ -12,6 +12,9 @@ type parent interface {
 	getHierarchy            () *Hierarchy
 	// canvas returns the canvas held by the parent.
 	getCanvas               () canvas.Canvas
+	// getInnerClippingBounds returns the area of the canvas that children
+	// can draw to.
+	getInnerClippingBounds  () image.Rectangle
 	// notifyMinimumSizeChange informs the parent that the minimum size of
 	// one of its children has changed.
 	notifyMinimumSizeChange (anyBox)