### 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...
for (int alternative_index = 0; alternative_index < atlas_source->get_alternative_tiles_count(tile_mapping.key); alternative_index++) {
					int alternative_id = atlas_source->get_alternative_tile_id(tile_mapping.key, alternative_index);
					int changed_id = -1;
					if (alternative_id == 0) {
						merged->create_tile(tile_mapping.value, atlas_source->get_tile_size_in_atlas(tile_mapping.key));
						int count = atlas_source->get_tile_animation_frames_count(tile_mapping.key);
						merged->set_tile_animation_frames_count(tile_mapping.value, count);
						for (int i = 0; i < count; i++) {
							merged->set_tile_animation_frame_duration(tile_mapping.value, i, atlas_source->get_tile_animation_frame_duration(tile_mapping.key, i));
						}
						merged->set_tile_animation_speed(tile_mapping.value, atlas_source->get_tile_animation_speed(tile_mapping.key));
						merged->set_tile_animation_mode(tile_mapping.value, atlas_source->get_tile_animation_mode(tile_mapping.key));
					} else {
						changed_id = merged->create_alternative_tile(tile_mapping.value, alternative_index);
					}
					
					TileData *src_tile_data = atlas_source->get_tile_data(tile_mapping.key, alternative_id);
					List<PropertyInfo> properties;
					src_tile_data->get_property_list(&properties);
					TileData *dst_tile_data = merged->get_tile_data(tile_mapping.value, changed_id == -1 ? alternative_id : changed_id);
					for (PropertyInfo property : properties) {
						if (!(property.usage & PROPERTY_USAGE_STORAGE)) {
							continue;
						}
						Variant value = src_tile_data->get(property.name);
						Variant default_value = ClassDB::class_get_default_property_value("TileData", property.name);
						if (default_value.get_type() != Variant::NIL && bool(Variant::evaluate(Variant::OP_EQUAL, value, default_value))) {
							continue;
						}
						dst_tile_data->set(property.name, value);