### Godot version

5ee983188de97ae027f9b9c1443438063f708a7e

### System information

Windows 11

### Issue description

Godot editor will throw an error and fail to generate preview icons for terrain tiles which use a base image which is not RGBA8 format.
`core\io\image.cpp:2788 - Condition "format != p_src->format" is true.`

### Steps to reproduce

- Import a non-RGBA8 image into Godot.
- Create a TileMap
- Create an associated TileSet
- Create a terrain for this tileset
- Add the image as an atlas
- Go to the paint tab
- Paint the terrain on the TileSet
- An error will occur any time terrains are edited in the TileSet editor or selected in the TileMap editor terrain tab.

### Minimal reproduction project

[tileset_terrain_bug.zip](https://github.com/godotengine/godot/files/13264213/tileset_terrain_bug.zip)

while (sources.has(next_source_id)) {
		next_source_id = (next_source_id + 1) % 1073741824; 
	};
}
int TileSet::add_source(Ref<TileSetSource> p_tile_set_source, int p_atlas_source_id_override) {
	ERR_FAIL_COND_V(!p_tile_set_source.is_valid(), TileSet::INVALID_SOURCE);
	ERR_FAIL_COND_V_MSG(p_atlas_source_id_override >= 0 && (sources.has(p_atlas_source_id_override)), TileSet::INVALID_SOURCE, vformat("Cannot create TileSet atlas source. Another atlas source exists with id %d.", p_atlas_source_id_override));
	ERR_FAIL_COND_V_MSG(p_atlas_source_id_override < 0 && p_atlas_source_id_override != TileSet::INVALID_SOURCE, TileSet::INVALID_SOURCE, vformat("Provided source ID %d is not valid. Negative source IDs are not allowed.", p_atlas_source_id_override));
	int new_source_id = p_atlas_source_id_override >= 0 ? p_atlas_source_id_override : next_source_id;
	sources[new_source_id] = p_tile_set_source;
	source_ids.push_back(new_source_id);
	source_ids.sort();
	TileSet *old_tileset = p_tile_set_source->get_tile_set();
	if (old_tileset != this && old_tileset != nullptr) {
		
		old_tileset->remove_source_ptr(p_tile_set_source.ptr());
	}
	p_tile_set_source->set_tile_set(this);
	_compute_next_source_id();
	sources[new_source_id]->connect_changed(callable_mp(this, &TileSet::_source_changed));
	terrains_cache_dirty = true;
	emit_changed();
	return new_source_id;
}
void TileSet::remove_source(int p_source_id) {