[Ohrrpgce] SVN: teeemcee/10431 Fix two apparent mistakes in gfx_sdl (one also in gfx_sdl2)

[subversion at HamsterRepublic.com]

teeemcee
2018-06-19 09:41:18 -0700 (Tue, 19 Jun 2018)
342
Fix two apparent mistakes in gfx_sdl (one also in gfx_sdl2)

-screenbuffer wasn't NULLed if gfx_sdl_set_screen_mode restarted the video
 subsystem
-the backends assumed that quitting just SDL_INIT_VIDEO subsystem would
 automatically shut down SDL_INIT_JOYSTICK too. (Maybe true, but not documented)

I don't know of any have not observed any
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U   wip/gfx_sdl.bas
U   wip/gfx_sdl2.bas