fix: Make gradient resizing refresh and not use cached textures

Avoid using cached textures for gradients, by refreshing the object on
resize.

Fixes #4608.

canvas/gradient.go

@@ -73,6 +73,15 @@ func (g *LinearGradient) Move(pos fyne.Position) {
 	repaint(g)
 }
 
+func (g *LinearGradient) Resize(size fyne.Size) {
+	if size == g.Size() {
+		return
+	}
+	g.baseObject.Resize(size)
+
+	g.Refresh()
+}
+
 // Refresh causes this gradient to be redrawn with its configured state.
 func (g *LinearGradient) Refresh() {
 	Refresh(g)
@@ -136,6 +145,15 @@ func (g *RadialGradient) Move(pos fyne.Position) {
 	repaint(g)
 }
 
+func (g *RadialGradient) Resize(size fyne.Size) {
+	if size == g.Size() {
+		return
+	}
+	g.baseObject.Resize(size)
+
+	g.Refresh()
+}
+
 // Refresh causes this gradient to be redrawn with its configured state.
 func (g *RadialGradient) Refresh() {
 	Refresh(g)