fix memleak of SDL_Surface objects

ee35f6a added a `surface& operator=(SDL_Surface* surf)` function that wrongly increased the refcounter of the passed surface so that it'd be  leaked later.

No we go back to the old behaviour where `surface& operator=(SDL_Surface* surf)` is handled by creating a temporary surface object that is calls then `surface& operator=(surface&& s)`

src/sdl/surface.hpp

@@ -52,12 +52,6 @@ class surface
 		return *this;
 	}
 
-	surface& operator=(SDL_Surface* surf)
-	{
-		assign_surface_internal(surf);
-		return *this;
-	}
-
 	surface& operator=(surface&& s) NOEXCEPT
 	{
 		free_surface();