### 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...
columns_editor_property->set_object_and_property(this, "next_line_after_column");
	columns_editor_property->update_property();
	columns_editor_property->connect("property_changed", callable_mp(this, &AtlasMergingDialog::_property_changed));
	atlas_merging_right_panel->add_child(columns_editor_property);
	
	Label *preview_label = memnew(Label);
	preview_label->set_text(TTR("Preview:"));
	atlas_merging_right_panel->add_child(preview_label);
	preview = memnew(TextureRect);
	preview->set_h_size_flags(Control::SIZE_EXPAND_FILL);
	preview->set_v_size_flags(Control::SIZE_EXPAND_FILL);
	preview->set_expand_mode(TextureRect::EXPAND_IGNORE_SIZE);
	preview->hide();
	preview->set_stretch_mode(TextureRect::STRETCH_KEEP_ASPECT_CENTERED);
	atlas_merging_right_panel->add_child(preview);
	select_2_atlases_label = memnew(Label);
	select_2_atlases_label->set_h_size_flags(Control::SIZE_EXPAND_FILL);
	select_2_atlases_label->set_v_size_flags(Control::SIZE_EXPAND_FILL);
	select_2_atlases_label->set_horizontal_alignment(HORIZONTAL_ALIGNMENT_CENTER);
	select_2_atlases_label->set_vertical_alignment(VERTICAL_ALIGNMENT_CENTER);
	select_2_atlases_label->set_text(TTR("Please select two atlases or more."));
	atlas_merging_right_panel->add_child(select_2_atlases_label);
	
	editor_file_dialog = memnew(EditorFileDialog);
	editor_file_dialog->set_file_mode(EditorFileDialog::FILE_MODE_SAVE_FILE);
	editor_file_dialog->add_filter("*.png");
	editor_file_dialog->connect("file_selected", callable_mp(this, &AtlasMergingDialog::_merge_confirmed));
	add_child(editor_file_dialog);
}