### Godot version

4.1

### System information

Windows 10.0.19045 - Vulkan (Forward+) - dedicated NVIDIA GeForce GTX 1060 (NVIDIA; 30.0.15.1403) - Intel(R) Core(TM) i7-7700HQ CPU @ 2.80GHz (8 Threads)

### Issue description


https://github.com/godotengine/godot/assets/2223172/3c8ee782-9d5f-42e3-a70f-d2aa59a233dd

We correctly clamp the size to 16x16 everywhere except here (probably).

### Steps to reproduce

1. Add a class with custom icon, e.g.
```GDScript
@icon("res://icon.svg")
extends Node
class_name Icon
```
2. Add it to scene
3. Open Script Editor and drag the node to script editor

### Minimal reproduction project

N/A
selected = nullptr;
	can_rename = p_can_rename;
	can_open_instance = p_can_open_instance;
	editor_selection = nullptr;
	if (p_label) {
		Label *label = memnew(Label);
		label->set_theme_type_variation("HeaderSmall");
		label->set_position(Point2(10, 0));
		label->set_text(TTR("Scene Tree (Nodes):"));
		add_child(label);
	}
	tree = memnew(Tree);
	tree->set_anchor(SIDE_RIGHT, ANCHOR_END);
	tree->set_anchor(SIDE_BOTTOM, ANCHOR_END);
	tree->set_begin(Point2(0, p_label ? 18 : 0));
	tree->set_end(Point2(0, 0));
	tree->set_allow_reselect(true);
	tree->add_theme_constant_override("button_margin", 0);
	add_child(tree);
	SET_DRAG_FORWARDING_GCD(tree, SceneTreeEditor);