sdl-video: Add function to free the surface backing a frame.
viewport: Add function to reinitialize landscape.

Is for example needed when a new map is loaded with different size.
mission: Move mission specifications to separate file.
game: Add functions to load mission specs and random maps.

This replaces the (badly named) game_load_random_map() which loaded mission maps with game_load_random_map() (which actually now loads a random map) and game_load_mission_map(). The code that is used by both initialization methods is moved to game_init_map().