[Ohrrpgce] SVN: teeemcee/12426 gfx_sdl2: fix window not recentering when the zoom is changed

[subversion at HamsterRepublic.com]

teeemcee
2021-09-12 04:45:09 -0700 (Sun, 12 Sep 2021)
304
gfx_sdl2: fix window not recentering when the zoom is changed

This regression was due to gfx_sdl2 not recreating the window, unlike gfx_sdl

Not recentering the window, especially after opening a game, was annoying and
ugly and I had more than one complaint about it.

(Copied from r12419 on wip branch)
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U   rel/hrodvitnir/gfx_sdl2.bas