[Ohrrpgce] SVN: teeemcee/12442 gfx_sdl2: fix viewport randomly getting shifted vertically when exiting

[subversion at HamsterRepublic.com]

teeemcee
2021-09-16 02:23:36 -0700 (Thu, 16 Sep 2021)
325
gfx_sdl2: fix viewport randomly getting shifted vertically when exiting fullscreen

Happened only on X11 (xfce4), and was due to a complicated SDL bug
https://github.com/libsdl-org/SDL/issues/4751

Also, I managed to reduce flicker a bit when switching between fullscreen and
windowed, but there's still a lot of it (on X11).
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U   wip/gfx_sdl2.bas