### Godot version

v4.2.dev3.official [013e8e3af]

### System information

Windows 11, Core i5-12400F, RX 6600

### Issue description

Any images without transparency become RGB8.
Any images with transparency become RGBA8.

When doing a TileSet Atlas Merge RGB8 images do not show up in merge dialog (just empty space) and do not merge.

![image](https://github.com/godotengine/godot/assets/962340/9cd422ce-2f86-41aa-b4f1-97bcba2db003)![Screenshot 2023-08-21 203922](https://github.com/godotengine/godot/assets/962340/905fe87d-c641-4c6e-bd0e-c47fe997c2ff)


### Steps to reproduce

- merge any solid image with no transparency

### Minimal reproduction project

https://github.com/rakkarage/TestTestTest333

EDIT: Added simpler example Test2.tscn, without margin, separation, and using real default size tiles...
if (p_action == "merge") {
		delete_original_atlases = true;
		editor_file_dialog->popup_file_dialog();
	}
}
bool AtlasMergingDialog::_set(const StringName &p_name, const Variant &p_value) {
	if (p_name == "next_line_after_column" && p_value.get_type() == Variant::INT) {
		next_line_after_column = p_value;
		_update_texture();
		return true;
	}
	return false;
}
bool AtlasMergingDialog::_get(const StringName &p_name, Variant &r_ret) const {
	if (p_name == "next_line_after_column") {
		r_ret = next_line_after_column;
		return true;
	}
	return false;
}
void AtlasMergingDialog::_notification(int p_what) {
	switch (p_what) {
		case NOTIFICATION_VISIBILITY_CHANGED: {
			if (is_visible()) {
				_update_texture();
			}
		} break;
	}
}
void AtlasMergingDialog::update_tile_set(Ref<TileSet> p_tile_set) {