Fix crash when CanvasBox draws with nil canvas

internal/system/box.go

@@ -203,7 +203,10 @@ func (this *box) SetPadding (padding tomo.Inset) {
 }
 
 func (this *box) SetRole (role tomo.Role) {
+	if this.role == role { return }
 	this.role = role
+	this.lastStyleNonce = -1
+	this.outer.recursiveReApply()
 }
 
 func (this *box) SetDNDData (dat data.Data) {

internal/system/canvasbox.go

@@ -29,7 +29,10 @@ func (this *canvasBox) Invalidate () {
 }
 
 func (this *canvasBox) Draw (can canvas.Canvas) {
+	if can == nil { return }
 	this.box.Draw(can)
+	if this.userDrawer != nil {
 		this.userDrawer.Draw (
 			can.SubCanvas(this.padding.Apply(this.innerClippingBounds)))
 	}
+}

x/canvas/draw.go

@@ -215,7 +215,7 @@ type fillingContext struct {
 }
 
 func (context *fillingContext) fillPolygonHotOpaque () {
-	for index := 0; index < len(context.boundaries); index += 2 {
+	for index := 0; index < len(context.boundaries) - 1; index += 2 {
 		left  := context.boundaries[index]
 		right := context.boundaries[index + 1]
 
@@ -240,7 +240,7 @@ func (context *fillingContext) fillPolygonHotOpaque () {
 }
 
 func (context *fillingContext) fillPolygonHotTransparent () {
-	for index := 0; index < len(context.boundaries); index += 2 {
+	for index := 0; index < len(context.boundaries) - 1; index += 2 {
 		left  := context.boundaries[index]
 		right := context.boundaries[index + 1]