Add pygame_sdl2.import_as_pygame().
Add pygame_sdl2.import_as_pygame().

This is a function that modifies sys.modules to replace the pygame
modules with pygame_sdl2, making it easier to port pygame code to
pygame_sdl2.

renpy/pygame_sdl2,renpy/pygame_sdl2,renpy/pygame_sdl2,renpy/pygame_sdl2