### 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...
set_hide_on_ok(false);
	
	set_ok_button_text(TTR("Merge (Keep original Atlases)"));
	get_ok_button()->set_disabled(true);
	merge_button = add_button(TTR("Merge"), true, "merge");
	merge_button->set_disabled(true);
	HSplitContainer *atlas_merging_h_split_container = memnew(HSplitContainer);
	atlas_merging_h_split_container->set_h_size_flags(Control::SIZE_EXPAND_FILL);
	atlas_merging_h_split_container->set_v_size_flags(Control::SIZE_EXPAND_FILL);
	add_child(atlas_merging_h_split_container);
	
	atlas_merging_atlases_list = memnew(ItemList);
	atlas_merging_atlases_list->set_fixed_icon_size(Size2(60, 60) * EDSCALE);
	atlas_merging_atlases_list->set_h_size_flags(Control::SIZE_EXPAND_FILL);
	atlas_merging_atlases_list->set_v_size_flags(Control::SIZE_EXPAND_FILL);
	atlas_merging_atlases_list->set_texture_filter(CanvasItem::TEXTURE_FILTER_NEAREST_WITH_MIPMAPS);
	atlas_merging_atlases_list->set_custom_minimum_size(Size2(100, 200));
	atlas_merging_atlases_list->set_select_mode(ItemList::SELECT_MULTI);
	atlas_merging_atlases_list->connect("multi_selected", callable_mp(this, &AtlasMergingDialog::_update_texture).unbind(2));
	atlas_merging_h_split_container->add_child(atlas_merging_atlases_list);
	VBoxContainer *atlas_merging_right_panel = memnew(VBoxContainer);
	atlas_merging_right_panel->set_h_size_flags(Control::SIZE_EXPAND_FILL);
	atlas_merging_right_panel->set_texture_filter(CanvasItem::TEXTURE_FILTER_NEAREST_WITH_MIPMAPS);
	atlas_merging_h_split_container->add_child(atlas_merging_right_panel);
	
	Label *settings_label = memnew(Label);
	settings_label->set_text(TTR("Settings:"));
	atlas_merging_right_panel->add_child(settings_label);
	columns_editor_property = memnew(EditorPropertyInteger);
	columns_editor_property->set_label(TTR("Next Line After Column"));