Fix ColorPicker always emitting color_changed on html submit

The color change was always emitted when the the modal was closed, even if it was exactly the same as before.

(cherry picked from commit 544727ac1ea7616c9f44116360d038cfcda5f763)

scene/gui/color_picker.cpp

@@ -195,12 +195,15 @@ void ColorPicker::_html_entered(const String &p_html) {
 		return;
 	}
 
-	float last_alpha = color.a;
+	Color previous_color = color;
 	color = Color::html(p_html);
 	if (!is_editing_alpha()) {
-		color.a = last_alpha;
+		color.a = previous_color.a;
 	}
 
+	if (color == previous_color) {
+		return;
+	}
 	if (!is_inside_tree()) {
 		return;
 	}