Support immediately update ui after resizing placeholder* texture

scene/resources/placeholder_textures.cpp

@@ -32,6 +32,7 @@
 
 void PlaceholderTexture2D::set_size(Size2 p_size) {
 	size = p_size;
+	emit_changed();
 }
 
 int PlaceholderTexture2D::get_width() const {
@@ -77,6 +78,7 @@ PlaceholderTexture2D::~PlaceholderTexture2D() {
 
 void PlaceholderTexture3D::set_size(const Vector3i &p_size) {
 	size = p_size;
+	emit_changed();
 }
 
 Vector3i PlaceholderTexture3D::get_size() const {
@@ -133,6 +135,7 @@ PlaceholderTexture3D::~PlaceholderTexture3D() {
 
 void PlaceholderTextureLayered::set_size(const Size2i &p_size) {
 	size = p_size;
+	emit_changed();
 }
 
 Size2i PlaceholderTextureLayered::get_size() const {