Removes an old `MovingSprite::has_found_sprite()` hack, which allowed getting whether a custom sprite has been found only the first time it's called. After the first call, `m_sprite_found` always equaled to `false`.

`MovingSprite::on_type_change` now checks if the call is initial, and if so, only sets the default sprite for the type, if a custom sprite hasn't just been loaded.

Closes SuperTux#1342.

…hops