Fix stretched shapes

plot.go

@@ -23,7 +23,7 @@ func (context plottingContext[T]) plot (center image.Point) {
 	square := context.square(center)
 	for y := square.Min.Y; y < square.Max.Y; y ++ {
 	for x := square.Min.X; x < square.Max.X; x ++ {
-		index := (y * context.stride + x) * context.width + context.offset
+		index := y * context.stride + x * context.width + context.offset
 		context.data[index] = context.color
 	}}
 }

polygon.go

@@ -65,7 +65,7 @@ func (this Image[T]) FillPolygon (fill []T, points ...image.Point) {
 				for offset, part := range fill {
 					for x := left; x < right; x ++ {
 						index :=
-							(y * this.Stride + x) *
+							y * this.Stride + x *
 							this.Width + offset
 						this.Pix[index] = part
 					}

rectangle.go

@@ -11,7 +11,7 @@ func (this Image[T]) FillRectangle (fill []T, rectangle image.Rectangle) {
 	for pos.Y = rectangle.Min.Y; pos.Y < rectangle.Max.Y; pos.Y ++ {
 	for offset, part := range fill {
 	for pos.X = rectangle.Min.X; pos.X < rectangle.Max.X; pos.X ++ {
-		index := (pos.Y * this.Stride + pos.X) * this.Width + offset
+		index := pos.Y * this.Stride + pos.X * this.Width + offset
 		this.Pix[index] = part
 	}}}
 }