### 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(p_property, p_value);
}
void AtlasMergingDialog::_generate_merged(Vector<Ref<TileSetAtlasSource>> p_atlas_sources, int p_max_columns) {
	merged.instantiate();
	merged_mapping.clear();
	if (p_atlas_sources.size() >= 2) {
		Ref<Image> output_image = Image::create_empty(1, 1, false, Image::FORMAT_RGBA8);
		
		Vector2i new_texture_region_size;
		for (int source_index = 0; source_index < p_atlas_sources.size(); source_index++) {
			Ref<TileSetAtlasSource> atlas_source = p_atlas_sources[source_index];
			new_texture_region_size = new_texture_region_size.max(atlas_source->get_texture_region_size());
		}
		
		Vector2i atlas_offset;
		int line_height = 0;
		for (int source_index = 0; source_index < p_atlas_sources.size(); source_index++) {
			Ref<TileSetAtlasSource> atlas_source = p_atlas_sources[source_index];
			merged_mapping.push_back(HashMap<Vector2i, Vector2i>());
			
			Vector2i atlas_size;
			for (int tile_index = 0; tile_index < atlas_source->get_tiles_count(); tile_index++) {
				Vector2i tile_id = atlas_source->get_tile_id(tile_index);
				atlas_size = atlas_size.max(tile_id + atlas_source->get_tile_size_in_atlas(tile_id));
				Rect2i new_tile_rect_in_atlas = Rect2i(atlas_offset + tile_id, atlas_source->get_tile_size_in_atlas(tile_id));
				
				for (int frame = 0; frame < atlas_source->get_tile_animation_frames_count(tile_id); frame++) {
					Rect2i src_rect = atlas_source->get_tile_texture_region(tile_id, frame);
					Vector2i new_position = new_tile_rect_in_atlas.position * new_texture_region_size;